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0U_1 62771
^ CQ
BRARY
LESSONS
IN
ELEMENTARY BIOLOGY
UY nil. I.A'IK
T. JEFFERY I’A^EK, D,.Si\. I^R.S.
m>FFs.sftK OF I'loi.oGV IN 1 Ml I NnrK-.ny oi- ihm-oin, m-av /faiami
WITH ONK HHXHKKI) AM) l^gc^TV-SKVKN 1 LL(\S'rKATIOJ)IS
MACMILLAN AX[> CO., LiMITKl;
ST. MARTIN’S .STRKKT, LONDON
1911
Rioiakii Ci av and Sons, l.iMirro
nRtNSWlCK hlKEET, STAMfORO STRJEr, S.K.,
AND BUNOAY, SUH-OIK.
/•'it’s/ Edition^ 1891.
Second KditioHy Eevtit-dy 1 193.
J/itrd Edition, Revised and Enlarged, 1897*
Reptmiedy 189?, iqoo, 1901, 1905, 1907, 1909, 191 1
PREFACE TO THE FIRST KDITtON
In his preface to the new edition of the well-known
Pmctkal Professor Huxley gives his reasons for
beginning the study of organised nature with the higher
forms of animal life, to the abandonment of liis earlier
method of working from the simpler to the more complex
organisnis, Ur says in effect that experience has taught
him the unwisdom of taking the beginner at once into the
new and strange region of microscojnc' life, and the advan-
tage of making him comrpence his studies with a subject of
which he is bound to know something* the elementary
anatomy and physiology of a vertebrate animal.
Most teachers will probably agree w'ith the general truth
of this opinion. I'he first few weeks of the beginner in
natural science are , so fully occupied in mastering an un-
familiar and difficult terminology and in acquiring- the art
of using his eyes and fingers, that he is simply incapable for
a time of grasping any of the principles of the science ; and,
this being the case, the more completely hi.s new work call
VI
FRKKACK TO FIRST EDITION
t)e connected with any knowledge of the subject, however
vague, he may already possess, the better for his progress.
On the other hand, the advantage to logical treatment of
proceeding from the sim[)le to the complex -of working
upwards from protists to the higher plants and animals is
so immense that it is not to be abandoned without very good
and suflicient reasons.
In my own exjK.rience I have found that the difficulty
may be largely met by a com[)romise, namely, by beginning
the work o(‘ the class l)y a comparative stmly of one of the
higher plants (flowering plant or fern) and of one of the
higluT animals (rabbit, frog, or crayfish). If there were no
limitations as to time, and if ifc were i)Ossible to avoid alto-
gether the valley of the shadow of the coming examination,
this preliminary work might be extended with advantage, and
made to include a fairly complete although elementary study
of animal physiology, with a minimum of anatomical detail,
and a somewhat extensive study of flowering jflants with
special reference to their physiology and to their relations
to the rest of nature.
In any case l)y the time this introductory work is over,
the student of average intelligence has overc.ome pre-
liminary ditficulties, and is ready to profit by the second
and more systematic |>art of the course in which organisms
are studied in the order of increasing complexity.
It is such a course of general elementary biology which
I have attempted to give in the following Lessons, my aim
having been to provide a book which may supply in the
study the place occupied in the laboratory by “ Huxley and
Martin,'' by giving the connected narrative which would be
I’RKFACK TO FIRST KDITION vii
out of place in a practical handbook. I also venture to
hope that the work may be of some use to students who
have studied zoology and l>otany as sejwate subjects, as
well as to that large class of workers whose services to
English science often receive but scant recognition I mean
amateur microscopists.
As to the general treatment of the subjec't I have been
guided by three principles. Firstly, that the main object of
teaching biology as pari of a liberal education is to familiarise
the student not so miu'h with the facts as with the ideas of
scieiK'e. Secondly, that such ideas are best understood, at
least by beginners, wlujn studied in connection with concrete
types of animals and plants. And, thirdly, that the types
chosen should illustrate without unnecessary complication
the particular grade of organisation they are intended to
typify, and that exceptional cases are out of place in an
elementary course.
'riie types have therefore been selected with a view of
illustrating all the more imf)ortant modifications of structure
and the chief physiological processes in plants and animals ;
and, by the occasional introduction of special lessons on
such subjects as biogenesis, evolution, iSce., the entire work
is so arranged as to give a fairly connected account of the
general principles of biology. It is in obedience to the last
of the principles just enunciated that I have described so
many of the Protozoa, omitted all but a brief reference to
the development of Hydra and to the so-called sexual pro-
cess in Penicillium, and described Nitella, instead of Chara,
and Polygordius instead of the earthworm. ITie last-named
.substitution is of course only made possible by the bpok
PREFACE TO FIRST EDITION
viii
being intended for the study and not for the laboratory, but
I feel convinced that the student who masters the structure
of Polygordius, even from figures and descriptions alone,
will be in a far better position to profit by a practical study
of one of the higher worms.
Lessons XXVII. and XXX.^ arc mere summaries, and can
only be read profitably by those who have studied the
organisms described, or allied forms, in some detail. Siu'h
al)stracts were however necessary to the plan of the book, in
order to show how all the higher animals and plants may
be desc'ribed, so to speak, in terms of Polygordius and of
the fern.
For many years I have been convinced of the urgent need
for a unification of terminology in biology, and have now
attempted to carry out a consistent scheme, as will be seen
by referring to the definitions in the glossary. Many of
Mr. Harvey Gibson’s suggestions are adopted, and three new
words are introduced— phyllula, gamobium, and agamo-
bium. I expect and perhaps deserve to be criticised, or,
what is worse, let alone, for the somewhat extreme step of
using the word ovary in its zoological sense throughout the
vegetable kingdom ; and for describing as the venter of the
pistil the so-called ovary of Angiosperms. I would only
beg my critics before finally pronouncing judgment to try
and look at the book, from the point of view of the begin-
ner, as a graduated course of instruction, and to consider
the effect upon the entire .scheme of using a term of funda-
mental importance in two utterly different senses.
A large proportion of the figures are copied either from
^ Sec Preface to the Third Edition, p. xi.
PREFACE TO FIRST EDITION
ix
original sources or from my own drawings - the latter when
no authority is mentioned. The majority, even of those
which have previously appeared in text-books, have been
specially engraved for the work, the draughtsman being
my brother, Mr. M. P. Parker. In order to facilitate
reference the illustrations referring to each subject have, as
far as possible, been grouped together, so that the actual is
considerably larger than the nominal number of figures.
Full descriptions are given instead of mere lists of reference-
letters : these will, I hope, be found useful as abstracts of
the subjects illustrated.
I have to thank my friends Mr. A. Dillon Bell and Pro
fessor J. H. Scott, M.D., for constant and valuable help in
criticising the manuscript. To Dr. Paul Meyer, of the
Zoological Station, Naples, I am indebted for specimens
of Polygordius ; and to Professor Sale, of this University,
Professor Haswell, of Sydney, Professor 'I'homas, of Auck-
land, and ProfeSvSors Howes and D. II. Scott, of South
Kensington, for important information and criticism on
special points. My brother. Professor W. Newton Parker,
has kindly promised to undertake a final revision for the
press.
Dunedin, N.Z.,
Auj^isi 1890 .
PREFACE TO THE THIRD EDITION
In the two former editions the “ Lessons ” practically
concluded witli Polygordius as an e\amj)le of tripoblastic
animals, and with the Fern as an example of vascular
plants, and the merest sketches of the higher groups of both
kingdoms were added (see Preface to the first edition,
p. viii). It has, however, been suggested to me from more
than one source, that the usfifulness of the book would be
increased by expanding these sketches into something more
comprehensible to the beginner.
This I have done in the present edition, with the result
that Lesson XXVII. of previous editions has been expanded
into I^essons XXVI. — XXIX., and Lesson XXX. into
Lessons XXXI 1. — XXXIV. The new matter is illustrated
with forty additional figures.
I have again to thank my brother, Prof. W. N. Parker,
for sacrificing much time in the labour of proof correcting.
Sepf ember 1896 .
TABLE OF CONTENTS
I'ACK
PREFACE TO THE FIRST EDITION V
PREFACE TO THE THIRD EDITION xi
LIST OF ILLUSTRATIONS . xix
LESSON L
AMg^BA I
LESSON II.
HA 2 MATOCOCCUS 23
LESSON III.
HETEROMITA 36
LESSON IV.
KUGLENA ^
LESSON V.
PROTOMYXA AND THE MYCETOZOA
49
xiv
TABLE OK CONTENTS
LESSON VI.
I'AUIi
A COMPARISON OF THE FOREGOING ORGANISMS WITH CERTAIN
CONSTITUENT PARTS OF THE HIGHER ANIMALS AND
PLANTS 56
ANIMAL AND PLANT CELLS 56
^llINUTE STRUCTURE AND DIVISION OF CELLS AND
NUCLEI 62
OVA OF ANIMALS AND PLANTS 69
LESSON VI T.
tACCHAROMYCES 7I
LESSON VIII.
BACTERIA . 82
r.ESSON IX.
BIOGENESIS AND ABIOGENKSIS . 95
HOMOGENESIS AND IIETEROGKNESIS . 102
LESSON X.
P ARAMCEC IUM . 106
STYLONYCIITA II6
OXYTRICHA . . ■ 120
LESSON XL
OPALINA
LESSON XXL
YOR TlCELL Aj 126
ZOOTHAMNIUM 135
TABLE OF CONTENTS xv
LESSON XIII.
I'ACtt
SPECIES AND THEIR ORIGIN : THE PRINCIPLES OF CLASSIFICA-
TION 137
LESSON XIV.
THE FORAMINIFKRA I4S
THE RADIOLARIA 152
THE DIATOMACEAi I55
LESSON XV.
MUCOR 158
I
LESSON XVI.
VAUCHERIA ... 169
CAULERPA . . 175
LESSON XVII.
rTHE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS . . J76
LESSON xvm.
PENICILLIUM . . 1S4
AGARICUS ... . . ... . . . I9I
LESSON XIX.
SPIROGYRA 194
LESSON XX.
.MONOSTROMA 201
ULVA 203
NITELLA 203
xvi TABLE OF CONTENTS
LESSON XXL
PAGE
HYDRA 218
LESSON xxir.
HYDROID POLYPES 234
BOUGAINVILLEA, &C 234
1)1 PH YES . 248
PORPITA 249
LESSON XXIII.
SPERMATOGENESIS AND OOGENESIS . . • . . . .... 253
THE MATURATION AND IMPREGNATION OE THE OVUM .... 258
THE CONNECTION BETWEEN UNICEL.LULAR AND DIPLOBLASTIC
ANIMALS 261
LESSON XXIV.
POLYGORDIUS 268
LESSON XXV.
POLYGORDIUS [continucif) 290
LESSON XXVI.
THE CHIEF DIVISIONS OF THE ANIMAL KINGDOM 304
THE STARFISH 306
LESSON XXVII.
THE CRAYFISH 3^8
LESSON XXVIII.
THE FRESH-WATER MUSSEL
348
TABLE OF CONTENTS xvii
LESSON XXIX.
PACE
THE DOGFISH . . 366
LESSON XXX,
MOSSES .... . ... 400
' LESSON XXXI.
FERNS ... 412
LESSON XXXII.
THE CHIEF DIVISIONS OF THE VKOETABIE KINGDOM . . 43I
EQUISETUM 434
SALVINIA ... 438
SEUaiNHI.l.A 442
LESSON XXXIII.
GVMNOSPERMS 447
LESSON XXXIV.
AHGIOSPERMS . . . ’ 4^1
SYNOPSIS ... 477
INDEX AND GLOSSARY
487
LIST OF ILLUSTRATIONS
Flfr PAGE
'i. various species 2
2. Protama'ba primiliva ... 9
3. Hu'matococcus pluvialis and IL lamslris 24
4. Heteromita rostrata 3 ^
5 . Etiglena viridis 45
6 . Protomoyxa aurantiaca 5 ®
7. Badhamia and Physarnm • • 53
8. Typical animal and vegetable cells 57
9. Animal and plant cells, detailed structure 62
10. Diagram illustrating the process of indirect cell -division 64
11. 0 \ 7 i oi Carniariita tirvX Gymnadenia ... .69
12. Saccharomyces cerevisiic . . .72
13. Bacterium termo . . . .... 83
14. Bacterium termOy showing flagella . . . 84
15 . Micrococcus .86
16. Bacillus siibtilis .87
1 7. Vibrio serpens. Spirillum tenue, and S, voliitans . 88
1 8 . Bacillus antkracis 90
19. Beaker with culture* tubes 100
20. Paramacium caudatum . .108
21. Para ma'cium caudatum, con)\x^2A\oTi 115
22. Stylonyckia myiilus 117
XX
LIST OF ILLUSTRATIONS
FIG. PAGE
23. Oxytniha flava 120
24. Opalina ranarum 1 22
25. VortUella ... 127
26. Zoofhammum arhttscu/a , ... 134
27. ZooihamtjiittUi various species 138
28. Diagram illustrating the Origin of the Species of Zootham*
num by Creation . 142
29. Diagram illustrating the Origin of the Species of Zootham-
ftiunt by Evolution 144
30. Ratal ia 149
31. Diagrams of Fomminifera 150
32. Aiveoiina qitoii .. 151
33. IJthodn'HS anuitlai'is 152
34. Actiuamma asteracanthioa , 153
35. Diagrams of a Diatom and shells of Navuuhi and Atthuo-
discus 156
Mucor miucdo and J/. stolonifcr 159
37. Moist Chamber 163
38. Vatic her ia 170
39. Catilcrpa scalpel li/or mis . 1 75
40. reuicilUum ^laucum 186
41. Ajit^rictts campestris 192
42. Spiro^ii}>ra 195
43. Monostroma buUosttm and J/, Icueratum 202
44. Nifclhiy general structure 204
45. NitcllUy terminal bud 209
46. Nifei/a, siwirmary 212
47. Nitella^ ovary 214
48. Chara^ pro-embryo 216
49. Hydra viridis and H. fusca^ external form ... . . 219
50. HydrUf minute structure . 223
51. Hydra^ ncmatocysl and nerve-cell 225
52. Hydra viridis ^ ovum 232
53. Bougaitwilka ramosa 235
54. Bucopelia^ portion of tentacle ... . 237
LIST OF ILLUSTRATIONS xxi
riG. I’ACB
55. Diagrams illustrating tlerivation of Medusa and Hydrantk . 241
56. Ettcopelia lampanitlaria^ muscle fibres and ccdls 243
57. Laomedea flexmsa and Eudendrium ramosum^ development. 247
58. Diphyes campamdata ... 250
59. Potpita padfica and P, meditcrranca . .... 251
60. Spermatogenesis in the Mole cricket . ... . 254
61. 0 \yxm oi Toxopucustes Iwidtts , . 257
62. Maturation and impregnation of the animal ovum .... 258
63. Pa>td<itdua monim . . 263
64. Vohox }(lobator . . ... 265
65. Volvox globator . . . 266
66. Polygordius ncapo/ifamts, external form 269
67. Polygorditts neapolitauusy anatomy 271
68. Polyy;ordiiis neapolitanusy nephridium . .... 282
69. PofygordztiSy diagram illustrating the relations of the nervous-
system 284
70. Polygoydius neapolitanusy reproductive organs 291
71. larva in the trochosjihere stage . 293
72. Diagram illustrating the origin of the trochospherc from the
gastrula 295
73. Polygordiits ueapolitauusy advanced trochospherc 297
74. Polygordiits neapolitanusy larva in a stage intermediate be-
tween the trochospherc and the adult 300
75. Starfish, ventral aspect 307
76. Starfish, diagrammatic sections 309
77. Starfish, digestive organs . . . 31 1
78. Starfish, water vascular sy.steni 313
79. Starfish, early stages in development 3*6
80. Starfish, development of bipinnaria larva 317
81. Crayfish, side view 3*9
82. Crayfish, principal appendages 322
83. Crayfish, diagrammatic sections . . • . . . 328
84. Crayfish, action of alxlominal muscles 330
85. Crayfish, leg, with muscles 331
86. Crayfish, dissection 333
xxii
LIST OF ILLUSTRATIONS
PIG. PAGE
87. Crayfi.sh, gills 336
88. Crayfish, <liagram of circulation of blood 340
89. Crayfish, early development 344
90. Crayfish, early cmhiyo in nauplius stage 345
91. Crayfish, later embryo 346
92. Mussel, side view, and shell 351
93. Mus.sel, diagrammatic sections 353
94. Mussel, di.ssection 356
95. Mussel, structure of gill 358
96. Mussel, circulatory system 361
9'"'. ]\Iusscl, advanced eml)r)o and free larva 364
98. Dogfish, side view 367
99. Dogfish, diagrammatic sections 370
100 Dogfish, skull 373
101. Dogfish, vertebr.e 376
102. Dogfish, pectoral arch 378
103. Dogfish, di.ssection 380
104. Dogfish, vascular system 385
104a. Dogfish, diagram of circulation 389
105. Dogfish, brain 392
106. Dogfish, early embryo 398
107. Dogfish, advanced embryo 399
108. Mosses, various genera, anatomy and histology 402
.109. I'unaria^ reproduction and development 406
up, Ptcris and Aspidium^ anatomy and histology 414
in. Ferns, various genera, reproduction and development . . . 424
1 12. P^qtthefitniy aerial shoot and spores 435
113. reproduction and development 437
1 14. Salvinia^ part of plant 439
115. AdrA/f/z/Vr, reproduction and dev'elopmcnt ........ 441
1 1 6. Seia^Neihii part of plant and sjiorangia 443
117. reproduction and development 445
1 18. Phtusy sections of stem 449
1 1 9. Gymnosperms, reproduction and development 453
120. PtMitSy stamen 455
LIST OF ILLUSTRATIONS xxiii
FIG. FACE
1 21. PimtSf carpel 45^
Zamia and Cycas^ reproductive organs 457
123. Lily, section of stem 462
124. Buttercup, structure of flower 465
125. Transition from petal to stamen 467
126. Angiosperms, reproduction and development 470
127. HeUebonts^ Campanula^ and Kihes^ flower 472
LESSONS
IN
ELEMENTARY BIOLOGY
LESSON I
'AM^BA
It is hardly possible to make a better beginning of the
systematic study of Biology than by a detailed examination
of a microscopic animalcule often found adhering to weeds
and other submerged objects in stagnant water, and known
to naturalists as Ainccha.
Amoebae are mostly invisible to the naked eye, rarely
exceeding one-fourth of a millimetre inch) in dia
meter, so that it is necessary to examine them entirely by
the aid of the microscope. They can be .seen and re-
cognised under the low power of an ordinary student’s
microscope which 'magnifies from twenty-five to fifty dia-
meters ; but for accurate examination it is necessary to
employ a far higher power, one in fact which magnifies
about 300 diameters.
Seen under this power, an Amoeba appears like a little
£ B
I GENERAL CHARACTERS 3
some formed of ectosarc only, others containing a core of endosarc.
The larger bodies in the endosarc arc mostly food-particles ( x 300).^
B. The same species, killed and stained with carmine to show the
numerous nuclei («//) ( x 300).
C. Amaba trotetis^ a living specimen, showing large irregular
pseudopods, nucleus (««), contractile vacuole (r. wr), and two food
vacuoles (/ mr), each containing a small infusor (see Lesson X.) whitjji
has been ingested as food. The letter a to the right of the figure in-
dicates the place where the protoplasm has united round the prey to
inclose the food vacuole. The contractile vacuole in this figure is
supposed to be seen through a layer of granular protoplasm, whereas
in the succeeding figures (D, H, and g) it is seen in optical section, and
therefore appears clear.
D. An encysted Amnc*ba, showing cell-wall or cyst {cy\ nucleus (//«),
clear contractile vacuole, and three diatoms (see Lesson XIV.) ingested
as food.
E. Amaba proiais, a living specimen, showing several large pseudo-
pods ipsil), single nucleus (««), and contractile vacuole (r. vac), and
numerous food -particles embedded in the granular endosarc ( x 330).
F. Nucleus of the same after staining, showing a ground substance
nuclear sap, containing deeply stained granules of chromatin, and
surrounded by a distinct membrane ( x 1010).
G. Amaba verrucosa, living specimen, showing winkled surface,
nucleus (««), large contractile vacuole (r. vac), and 4veral ingested
organisms ( x 330). » * ^
II. Nucleus of the same, stained, showing the chromatin aggregated
in the centre ( x 1010).
1. Ama'ba protem, in the act of multiplying by binary fission
(X 500).
(A, H, E, F, G, and H after Gruber ; c and I after Leidy ; D after
Howes.)
shapeless blob of jelly, nearly or quite colourless. The
central part of it (Fig. i, A, c, and e) is granular and semi-
transpwent — something like ground glass — while surround-
ingThis inner mass is a border of perfectly transparent and
colourless substance. So clear, indeed, is this outer layer
that it is easily overlooked by the beginner, who is apt to take
the granular internal substance for the whole Amceba. If
in any way the creature can be made to turn over, or if a
number of specimens are examined in various positions,
these two constituents will always be found to have the
1 A number preceded by the sign of multiplication indicates the
number of diameters to which the object is magnified.
B 2
4
AMCEBA
LESS.
same relations, whence we conclude that an Amoeba con-
sists of a granular substance the endosarc^ completely
surrounded by a clear transparent layer or edosarc.
One very noticeable thing about Amoeba is that it is never
of quite the same shape for long together. Often the
changes of form are so slow as to be almost imperceptible,
like the movements of the hour-hand of a watch, but by
examining it at successive intervals the alteration becomes
perfectly obvious, and at the end of half an hour it will
probably have altered so much as to be hardly like the
same thing.
In an active specimen the way in which the changes of
form are brought about is easily seen. At a particular
point the ectosarc is pushed out in the form of a small
pimple-like elevation (Fig. i, a , left side) : this increases in
size, still consisting of ectosarc only, until at last granules
from the cndosarc stream into it , and the projection or
psendflpod ( a , c , k , psd) comes to have the same structure
as the rest of the Amoeba. It must not be forgotten that
t he animal does not alter perceptibly in volume during
the process, every pseudopod thus protruded from one part
of the body nece.ssitating the withdrawal of an equal volume
for some other part.
This peculiar mode of movement may be illustrated by
taking an irregular lump of clay or putty and squeezing it
between the fingers. As it is compressed in one direction
it will elongate in another, and the squeezing process may
be regulated so as to cause the protrusion of comparatively
narrow portions from the solid lump, vrhen the resemblance
to the movements described in the preceding paragraph will
be fairly close. Only it must be borne in mind that in
Amoeba there is no external compression, the “ squeezing ”
being done by the animalcule itself.
COMPOSITION OF PROTOPLASM
5
The occurrence of these movements is alone sufficient to
show that Amoeba is an organism or living thing, and no
mere mass of dead matter.
The jelly-like substance of which Amoeba is composed
is called protoplasm. It is shown by chemical analysis ^
to c onsistliiainfir of certain substances known as proteids ^
bodies of extreme complexity in chemical constitution, the
most familiar example of which is white of egg or albumen.
They are compounds of carbon, hydrogen, oxyg en, nitroge iL
and sulphur, the five elements being combined in the
following proportions :
Carbon .
. from 51*5 to 54*5 per cent.
Hydrogen
• .. 69 ..
7*3 >> >>
Oxygen
• .. 20’9 ..
23*5 »»
Nitrogen
. .. I5'2 ..
0 „ ,,
Sulphur
• .. o'3 »
2 0 >f I)
Besides proteids, protoplasm contains small proportions
of mineral matters, especially phosphates and sulphates of^
potassium, calcium, and magnesium. It also contains a ^
considerable quantity o f water which, being as essential a
constituent of it as the proteids and the mineral salts, is
called water of organization.
Protoplasm is dissolved by prolonged treatment with weak
acids or alkalies. Strong alcohol coagulates it, />., causes it
to shrink by withdrawal of water and become comparatively
hard and opaque. Coagulation is also produced by raising
the temperature to about 40 '' C . ; the reader will remember
how the familiar proteid white of egg is coagulated 5nd
rendered hard and opaque by heat.
^ Accurate analyses of the protoplasm of Amoeba have not been
made, but the various micro chemical tests which can be applied to it
leave no doubt that it agrees in all essential respects with the protoplasm
of other oiganisms, the composition of which is known (see p. 7).
6
AMCEBA
LESS,
There is another important property of proteids which is
tested by the instrument called a dialvser . This consists
essentially of a shallow vessel, the bottom of which is made
of bladder, or vegetable parchment, or some other organic
(animal or vegetable) membrane. If a solution of sugar or
of salt is i)laced in a dialyser and the instrument floated in a
larger vessel of distilled water, it will be found after a time
that some of the sugar or salt has passed from the dialyser
into the outer vessel through the membrane. On the other
hand, if a solution of white of egg is placed in the dialyser
no such transference to the outer vessel will take place.
The dialyser thus allows us to divide substances into
two classes : crystalloids — ~^o called because most of them,
like salt and sugar, are capable of existing in the form of
crystals — which, in the ^tc of solution, will diffuse thrpugh
an organic membrane ; and colloids or gluc-like substances
which will not diffuse. Protoplasm, like the proteids of
which it is largely composed, is a colloid, that is, is non-
diffusible. It has a slightly alkaline reaction .
Another character of proteids is their histability . A
lump of salt or of sugar, a piece of wood or of chalk, may
be preserved unaltered for any length of time, but a proteid
if left to itself very soon begins to decompose ; it acquires
an offensive odour, and breaks up into simpler and simpler
compounds, the most important of which are water (HgO),
carbon dioxide or carbonic acid (COg), ammonia (NHg),
and sulphuretted hydrogen In this character of
instability or readiness to decompose protoplasm notoriously
agrees with its constituent proteids ; any dead organism will,
^ For a more detailed account of the phenomena of putrefaction see
Lesson VIII., in which it will be seen that the above statement as to
the instability of (dead) proteids requires qualification j as a matter of
fact they decompose only in the presence of living Bacteriat
I CHARACTERS OF THE NUCLEUS 7
unless special means are taken to preserve it, undergo more
or less speedy decomposition.
Many of these properties of protoplasm can hardly be
verified in the case of Amoeba, owing to its minute size and
the difficulty of isolating it from other organisms (water-
weeds, &c.) with which it is always associated ; but there
are some tests which can be readily applied to it while
under observation beneath the microscope.
One of the most striking of these micro chemical tests
depends upon t he avidity with wffiich protoplasm takes up
certain colouring matters. If a drop of a neutral or slightly
alkaline solution of carmine or logwood, or of some aniline
dye, or a weak solution of iodine, is added to the water con-
taining Amceba, the animalcule is killed, and at the same
time becomes more or less deeply stained.
The staining is. however, not uniform . The endosarc,
owing to the granules it contains, appears darker than the
ectosarc, and there is usually to be seen, in the endosarc, a
rounded spot more brightly stained than the rest. This
structure, which can sometimes be seen in the living Amoeba
(Fig. I, c, E, and g, nu), while frequently its presence is
revealed only by staining (comp, a and b), is called the
nucleus.
But when viewed under a sufficiently high power, the
nucleus itself is seen to be unequally stained. It has lately
been shown, in many Amoebae, to be a globular body, en-
closed in a very delicate membrane, and made up of two
constituents, one of which is deeply stained by colouring
matters, and is hence called chromatin , while the other, the
nuclear sap or achromatin . takes a lighter tint (Fig. i, v).
The relative arrangement of chromatin and sap varies
in different Amoebae: sometimes there are granules of
chromatin in an achromatic ground substance (f); some*
8 AMCEBA LESS.
times the chromatin is collected towards the surface or
periphery of the nucleus ; sometimes, again, it becomes
^KJ^cgatcd in the centre (c;, h). One or more smaller
bodies, or nucleoli^ may also be present in the nucleus .
which is then distinguished as the nucleolus.
When it is said that Amoebae sometimes have one kind of
nucleus and sometimes another, it must not be inferred that
the same animalcule varies in this respect. What is meant
is that there are found both in fresh and salt water many
kinds or species of Amoeba which are distinguished from one
another, amongst other things, by the character of their
nuclei, just as the various species of Felis — the cat, lion,
tiger, lynx, &c. — are distinguished from one another, amongst
other things, by the colour and markings of their fur.
According to the method of binomial nomenclature intro-
duced into biology by Linnoeus, the same generic name
is applied to all such closely allied species, while each is
specially distinguished by a second or specific name of its
own. Thus under the genus Amoeba are included Amoeba
Proteus (Fig. i, c, e, and f), with long lobed pseudopods and
a nucleus containing evenly-disposed granules of chromatin;
A. ouarta (a and b), with short pscudopods and numerous
nuclei ; A, verrucosa (o and h) w'ith crumpled or folded
surface, no well-marked pseudopods, and a nucleus with a
central aggregation of chromatin substance ; and many
others.
Besides the nucleus, there is another structure frequently
visible in the living Amoeba. This is a clear, rounded space
in the protoplasm (c, e, and o, c. vac\ which periodically
disappears \nth a sudden contraction and then slowly
reappears, its movements reminding one of the beating of a
minute colourless heart. It is called the contractile vacuole l
and consists of a cavity containing a watery fluid.
I
MORPHOLOGY AND PJ D Sl< >1 ( H'.V
9
'Occasionally Amoebae — or more strictly yXimeha like
organisms — are met with which show neither nucleus ^ nor
contractile vacuole , a nd are therefore placed in th e S(']eiratc
genus ProtiDmvba (Fig. 2). They may be looked upon as
the simplest of living things*
Fku 2-~Protanhvha pritnUiva y A, ]{, llic same s^jcciinen djawii at
short intervals of tune, showing changes ul foiin.
C— E. Three stages in the process of hinaiy fission. (After Haeckel.)
The preceding paragratihs may be summed u]) by saying
that Amoeba is a mass of protoplasm jrroduced into temi)o-
rary processes or pscudojrods, divisible into ectosarc and
endosarc, and containing a nucleus and a contractile vacuole :
that the nucleus consists of two substances, chromatin and
nuclear sap, enclosed in a distinct membrane : and that the
contractile vacuole is a mere cavity in the prototrlasm r'on-
taining fluid. All these facts come under tlie head of
Morphology^ the division of biology which treats of form
and structure ; we must now study the Physiology of our
animalcule — that is, consider the actions or fufictions it is
capable of performing.
First of all, as we have already seen, it moves, the move-
ment consisting in the slow protrusion and withdrawal of
pseudopods. This may be expressed generally by saying
^ Judging from the analogy of the Infusoria it seems very ])roljal)lL:
that such a])parcntly non-nucicate forms as Protamajba contain chroma-
tin diffused in the form of minute granules throughout their substance
[see end of X>esson X., p. 120), or that they are forms which liave lost
their nuclei.
lO
AMCEBA
LESS,
that Amccha is contractile^ or that it exhibits co7itractility.
But here it must be borne in mind that contraction docs
not mean the same thing in biology as in physics. When
it is said that a red-hot bar of iron contracts on cooling,
what is meant is that there is an actual reduction in
volume, the bar becoming smaller in all dimensions. But
when it is said that an Aniceba contracts, what is meant is
that it diminishes in one dimension while increasing in
another, no perceptible alteration in volume taking place :
each time a pscudopod is protruded an equivalent volume
of protoplasm is withdrawn from some other part of the
body.
We may say then that c ontractility is a function of the
prQto])lasm of Amceba — that is, that it is one of the actions
which the protoplasm is capable of performing.
A contraction may arise in one or other of two ways. In
most cases the movements of an Aniceba take jilace without
any obvious external cause ; they are what would be called
in the higher animals voluntary movements — movements
dictated by the will and not necessarily in response to any
external stimulus. Such movements are called spontaneous
or automatic . On the other hand, movements may be in-
duced in Amoeba by external stimuli, by a sudden shock,
or by coming into contact with an object suitable for food :
such movements are the result of irriiabilitv of the proto -
plasm, which is thus both’ automatic and irritable — that is,
its contractility may be set in action either by internal or by
external stimuli.*
Under certain circumstances an Amceba temporarily loses
its power of movement, draws in its pseudopods, and
becomes a globular mass around which is formed a thick,
shelldike coat, called the cyst or cell-ivail (Fig. i, d, ey).
The composition of this is not known ; it is certainly not
I
MODE OF FEEDING
II
protoplasmic, and very i )robably consists of some nitrogenous
substance allied in composition to horn and to the chitin
which forms the external shell of Crustacea, insects, &c.
After remaining in this encysted condition for a time, the
Amceba escapes by the rupture of its cell- wall, and resumes
active life.
Very often an Amoeba in the course of its wanderings
comes in contact with a still smaller organism, such as a
diatom (see Lesson XIV., Fig. 35) or a small infusor (see
Lessons X. — XIL). AVhen this hai)pens the Amceba may
be seen to extend itself round*‘the lesser organism until the
latter becomes sunk in its ])rotoplasm in much the same way
as a marble might be pressed into a lump of clay (Fig. i,
c, rtf). The diatom or other organism becomes in this way
completely enclosed in a cavity or food-vacuote (/. vac\
which a lso contains a small cniantitv of water necessarily in-
cluded with the prey. The latter is taken in by the Amceba
as food : so that another function performed by the animal-
cule is the reception of food, the first step in the process of
nutrition. It is to be noted that the reception of food takes
place in a particular way, viz, by iny;estio 7 i — i.e, it is enclosed
raw and entire in the living protoplasm. It has been noticed
that Amoeba usually ingests at its hinder end — that is, the
end directed backwards in progrc.ssion.
Having thus ingested its prey, the Amoeba continues its
course, when, if carefully watched, the swallowed organism
will be seen to undergo certain changes. Its protoplasm
is slowly dissolved; if it contains chlorophyll — the green
colouring matter of plants — this is gradually turned to brown ;
and finally nothing is left but the case or cell-wall in which
many minute organisms, such as diatoms, are enclosed.
Finally, the Amoeba, as it creeps slowly on, leaves this empty
cell- wall behind, and thus gets rid of what it has no further
12
AMCEBA
LESS,
use for. It is thus able to ins^est living organisms as food ;
to dissolve or dmst their protoplasm ; and to emt or get
rid of any insoluble matorials they may contain. Note
that all this is done without either ingestive aperture (mouth),
digestive cavity (stomach), or egestive aperture (anus) ; the
food i^ simply taken in by the flowing round it of protoplasm,
digested as it lies enclosed in the protoplasm, and the useless
part got rid of by the Amceba flowing away from it.
It has just been said that the protoplasm of the prey is
dissolved or digested* : we must now consider more particu-
larly what this means.
The stomachs of the higher animals — ourselves, for
instance — i)roduce in their interior a fluid called gastric
juice. When this fluid is brought into contact with albumen
or any other proteid a remarkable change takes place . The
proteid is dissolved and at the .same time rendered diffusible,
so as to be capable, like a solution of salt or sugar, of passing
through an organic membrane (see p. 6). The diffusible
proteids thus formed by the action of gastric juice upon
ordinary ])rotcids are called peptoties : the transformation is
effected through the agency of a constituent of the gastric
juice called pepsin .
Hiere can be little doubt that the protoplasm of Amceba
is able to convert that of its prey into a soluble and diffusible
form by the agency of some substance analogous to pepsin,
and that the dissolved matters diffuse through the body of
the Amoeba until the latter is, as it were, soaked through
and through with them. Under these circumstances the
Amoeba may be compared to a sponge which is allowed to
absorb water, the sponge itself representing the living proto-
plasm, the water the solution of proteids which permeates it.
It has been proved by experiment that proteids are the only
class of food which Amoeba can make use of : it is unable to
1
GROWTH
13
digest either starch or fat — two very important constituents
of the food of the higher animals. Mineral matters must,
however, be taken with the food in the form of a weak
watery solution, since the water in which the animalcule
lives is never absolutely pure.
The Amoeba being thus permeated, as it were, with a
nutrient solution, a very important process takes place. The
elements of the solution, hitherto arranged in the form of
peptones, mineral salts, and water, become rearranged in
such a way as to form new particles of living protoplasm,
which are deposited among the pre-existing j)articles. In a
word, the food is assimilated or converted into the actual
living substance of the Amoeba.
One effect of this formation of new protoplasm is obvious ;
if nothing happens to counteract it, the Amoeba must
the increase in size being brought about in much the same
way as that of a heap of stones would be by continually
thrusting new pebbles into the interior of the heap. This
mode of growth— by the interposition of new particles among
old ones — is called growth by intussusception , and is very
characteristic of the growth of protoplasm. It is necessary
to distinguish it, because there is another mode of growth
which is characteristic of minerals and occurs also in some
organized structures. A crystal of alum, for instance,
suspended in a strong solution of the same substance, grows ;
but the increase is due to the deposition of successive layers
on the surface of the original crystal, in much the same way
as a candle might be made to grow by repeatedly dipping it
into melted grease. This can be proved by colouring the
crystal with logwood or some other dye before suspending
it, when a gradually-increasing colourless layer will be
deposited round the coloured crystal : if growth took place
by intussusception we should have a gradual weakening
M
AMOEBA
LESS.
of the tint as the crystal increased in size. This mode of
growth “ by the deposition of successive layers — is called
growth by accretion .
It is probable that the cyst of Amoeba referred to above
(p. ij) grows by accretion . Judging from the analogy of
other organisms it would seem that, after rounding itself off,
the surface of the si)hcre of protoplasm undergoes a chemi-
cal change resulting in the formation of a thin superficial
layer of non-protoplasmic substance. The jirocess is re-
peated, new layers being continually deposited within the
old ones until the cell-wall attains its full thickness. The
cyst is therefore a substance separated or s ec reted from the
protonlasm ; it is the first instance we have met with of a
product of secretion.
From the fact that Amoeba rarely attains a greater dia-
meter than I mm., it follows that something must happen to
counteract the constant tendency to grow, which is one of
the results of assimilation. We all know what happens in
our own case : if we take a certain amount of exercise —
w'alk ten miles or lift a series of heavy weights — we undergo
a loss of substance manifested by a diminution in weight
and by the sensation of hunger. Our bodies have done a
certain amount of work, and have undergone a proportionaf
amount of waste, just as a fire every time it blazes up
consumes a certain weight of coal.
Precisely the same thing happens on a small scale with
Amoeba. Every time it thrusts out or withdraws a pseudo-
pod, every time it contracts its vacuole, it docs a certain
amount of w’ork — moves a definite weight of protoplasm
through a given space. And every movement, however slight,
is accompanied by a proportional waste of substance, a cer-
tain fraction of the protoplasm becoming oxidized, or in other
words undergoing a process of low temperature combustion.
I POTENTIAL AND KINETIC ENERGY i?
When we say that any combustible body is burnt what we
usually mean is that it has combined with oxygen, forming
certain products of combustion due to the chemical union
of the oxygen with the substance burnt. For instance, when
carbon is burnt the product of combustion is carbon dioxide
or carbonic acid (C + 0 ^ == CO^) *. when hydrogen is burnt,
water (H2 + O === The products of the slow com-
bustion which our own bodies are constantly undergoing
are these same two bodies — c arbon dioxide given off mainly
i n the air breathed out , and water ^iven off mainly in the
form of perspiration and urine — together with two com-
pounds containing nitrogen, urea (CH4N0O) and uric acid
(CgH^^N^Oy), both occurring mainly in the urine. In some
animals urea and uric acid are replaced by other comuounxis
such as guanin (Cr,Hr,NgO), but it may be taken as proved
that ill all living things the* products of combustion are
carbon dioxide, water, and some nitrogenous substance of
simpler constitution than proteids, and allied to the three
just mentioned.
With this breaking down of proteids the vital activities of
all organisms are invariably connected. Just as useful
mechanical work may be done by the. fall of a weight from
a given height to the level of the ground, so the w^ork done
by the organism is a result of its complex proteids falling,
so to speak, to the level of simpler substances. In both
instances potential energy or energy of position is converted
into kinetic or actual energy.
In the particular case under consideration we have to rely
upon analogy and not upon direct experiment. We may,
however, be quite sure that the products of combustion
or waste matters of Amoeba include carbon dioxide, water,
and some comparatively simple (as compared with proteids)
compound of nitrogen.
i6 AMCEBA LESS,
These waste matters or excretory products are given off
partly from the general surface of the body, but partly, it
would seem, through the agency of the contractile vacuole.
It appears that the water taken in with the food, together in
all probability with some of that formed by oxidation of
the protoplasm, makes its way to the vacuole, and is ex-
pelled by its contraction. We have here another function,
performed by Amoeba, that of excretion, or the getting rid
of waste matters.
In this connection the reader must be warned against a
possible misunderstanding arising from the fact that the
word excretion is often used in two senses. We often hear,
for instance, of solid and liquid “excreta.” In Amoeba
the solid excreta, or more correctly fcpces, consist of such
things as the indigestible cell-walls, starch grains, &:c., of the
organisms upon which it feeds ; but the rejection of these
is no more a process of excretion than the spitting out of
a cherry-stone, since they are simply parts of the food
which have never been assimilated — never formed part and
parcel of the organism. True excreta, on the other hand,
are invariably products of the waste or decomposition of
protoplasm.^
The statement just made that the protoplasm of Amoeba
constantly undergoes oxidation presupposes a constant sup-
ply of oxygen. The water in which the animalcule lives
invariably contains that gas in solution : on the other hand,
as we have seen, the protoplasm is continually forming
carbon dioxide. Now when two gases are separated from
one another by a porous partition, an interchange takes place
between them, each diffusing into the space occupied by the
^ In the higher animals the distinction between excreta and fseces is
complicated by the fact that the latter always contain true excretory
products derived from the epithelium of the intestine and its glands.
MtiTACOLt^lM
other. The same process of gaseous diffusion is continually
going on between the carbon dioxide in the interior of
Amoeba and the oxygen in the surrounding water, the ])roto-
jdasm acting as the porous partition. In this way the carbon
dioxide is got rid of, and at the same time a supi)ly of
oxygen is obtained for further combustion.
The taking in of oxygen might be looked upon as a kind
of feeding process, the food being gaseous instead of solid
or liquid, just as we might speak of “ feeding ” a fire both
with coals and with air. Moreover, as we have seen, the
giving out of carbon dioxide is a process of excretion. It
is, however, usual and convenient to speak of this process
of exchange of gases as respiration or breathing, which is
therefore another function performed by the protoidasm of
Amoeba.
The oxidation of protoplasm in the body of an organism,
like the combustion of wood or coal in a fire, is accompanied
by an evolution of heat. That this occurs in Amceba can-
not be doubted, although it has never been proved. The
heat thus generated is, however, constantly being lost to the
surrounding water, so that the temperature of Amoeba, if we
could but measure it, would probably be found, like that of
a frog or a fish, to be very little if at all above that of the
medium in which it lives.
We thus see that a very elaborate series of chemical pro-
cesses is constantly going on in the interior of Amoeba.
These processes are divisible into two sets ; those which
begin with the digestion of food and end with the manufac-
ture of living protoplasm, and those which have to do with
the destruction of protoplasm and end with excretion.
The whole series of iprocesses are spoken of collectively
as metabolism . We have, first of all, digested food diffused
through the protoplasm and finally converted into fresh
c
AMCEBA
LttSS.
iS
living protoplasm : this is the process of constructive meta -
bolism or anabolism . Next we have the protoplasm, gradually
breaking down and undergoing conversion into excretory
products ; tnis is the process of destructive metabolism or
katabolism, 'rhere can be little doubt that both are pro-
cesses of extreme complexity : it seems probable that
after the food is once dissolved there ensues the successive
formation of numerous bodies of gradually increasing
com})lexity { anabolic mesostates or an as fates ), culminating
in protoplasm ; and that the protoplasm, when once formed,
is decomposed into a series of substances of gradually
diminishing comi)lexity ( katabolic mesostates or katas tatesX
the end of the series being formed by the comparatively
sim])le products of excretion. The granules in the endosarc
arc probably to be looked upon as various mesostates
imbedded in the j)rotoi)lasm proper.
Living protoplasm is thus the most unstable of substances ;
it is never precisely the same thing for two consecutive
seconds: it “decomposes but to recompose,” and recom-
poses but to dccomi)ose ; its existence, like that of a water-
fall or a fountain, depends upon the constant flow of matter
into it and away from it.
It follows from what has been said that if the income of
an Amceba, i.e,, the total weight of substances taken in (food
plus oxygen plus water) is greater than its expenditure or
the total weight of substances given out (faeces plus excreta
proper plus carbon dioxide) the animalcule will grow : if less
it will dwindle away : if the two are equal it will remain of
the same weight or in a state of physiological equilibrium.
We see then that the fundamental condition of existence
of the individual Amceba is that it should be able to form
new protoplasm out of the food supplied to it. But some-
I REPRODUCTION 19
thing more than this is necessary. Amoebre are subject to
all sorts of casualties ; they may be eaten by other organ-
isms or the pool in which they live may be dried up ; in one
way or another they are constantly coming to an end.
From which it follows that if the race of AriKebae is to be
preserved there must be some provision by which the
individuals composing it are enabled to ])roduce new in-
dividuals. In other words Amoeba must, in addition to its
other functions, perform that of reproduction.
An Amoeba reproduces itself in a very simple way. The
nucleus first divides into two : then the whole organism
elongates, the two nuclei at the same time travelling away
from one another : next a furrow appears across the middle
of the drawn out body between the nuclei (Fig. 1, i ; fig. 2,
c, d) : the furrow deepens until finally the animalcule sepa-
rates into two separate Amoebae (Fig. 2, e), which hence-
forward lead an independent existence.
This, the simplest method of reproduction known, is
called simple or binary fission. Notice how strikingly dif-
ferent it is from the mode of multiplication with which we
are familiar in the higher animals. A fowl, for instance,
multiplies by laying eggs at certain intervals, in each of
which, under favourable circumstances, and after a definite
lapse of time, a chick is developed : moreover, the parent
bird, after continuing to produce eggs for a longer or shorter
time, dies. An Amoeba, on the other hand, simply divides
into two Amoebje, each exactly like it.self, and in doing
so ceases to exist as a distinct individual. Instead of the
successive production of offspring from an ultimately dying
parent, we have the simultaneous' production of offspring
by the division of the parent, which does not die, but
becomes simply merged m its progeny. There can be no
better instance of the fact that reproduction is discontinuous
growth.
c 2
26
AMCEBA
LESS.
From this it seems that an Amoeba, unless suffering
a violent death, is i)ractically immortal, since it divides into
two completely organised individuals, each of which begins
life with half of the entire body of its parent, there being
therefore nothing left of the latter to die. It is possible,
however, judging from the analogy of the Infusoria (see
Lesson X.) that such organisms as Amoeba cannot go on
multiplying indefinitely by simple fission, and that occasion-
ally two individuals come into contact and undergo complete
fusion. A conjugation of this kind has been observed in
Amoiba, but has been more thoroughly studied in other forms
(see Lessons IIL, X., XIL). Whether it is a necessary
condition of continued existence in our animalcule or* not,
it appears certain that ** death has no place as a natura l
recurrent phenomenon ” in that organism.
Amceba may also be propagated artificially. If a speci-
men is cut into pieces each fragment is capable of develop-
ing into a complete animalcule provided it contains a
portion of nuclear matter, but not otherwise. From this it
is obvious that t he nucleus exerts an influence of the utmos t
importance over the vital processes of the organism.
If an Amoeba does happen to be killed and to escape
being eaten it will undergo gradual decomposition^ becoming
converted into various simple substances of which carbon
dioxide, water, and ammonia are the chief. (See p. 91.)
In conclusion, a few facts may be mentioned as to the
conditions of life of Amoeba — the circumstances under
which it will live or die, flourish or otherwise^.
In the first place, it will live only within limit s , of
temperature. In moderately warm weather the temperature
to which It is exposed may be taken as about 15° C. If
gradually warmed beyond this point the movements aTfirst
I
CONDITIONS OF LIFE
21
show an increased activity, then become more and more
sluggish, and at about 3o°--35° C. cease altogether, re-
commencing, however, when the temperature is lowered.
iF the' heating is continued up to about 40° C. the animal-
cule is killed by the coagulation of its protoplasm (see p. 5) :
it is then said to suffer heat rigor or death-stiffening pro-
duced by heat. Similarly when it is cooled below the
ordinary temperature the movements become slower and
slower, and at the freezing point (0° C.) cease entirely^
But freezing, unlike over-heating, docs not kill the pro*-
toplasm, but only renders it temporarily inert ; on thawingj
the movements recommence. We may therefore distinguish
an optimum temperature at which the vital actions arc carried
on with the i^rcatest activity ; maximum and minimum tem-^
peratures above and below which respectively they cease ;!
and an tiltra- maximum temperature at which death ensues.
There is no definite ultra-minimum temperature known in
the case of Amceba.
The quantity of water present in the protoplasm — as water
of organization (see p. 5) — is another matter of importance.
The water in which Amoeba lives always contains a certain
percentage of salts in solution, and the protoplasm is
affected by any alteration in the density of the surrounding
medium ; for instance, by replacing it by distilled water and
so reducing the density, or by adding salt and so increasing
it. The addition of common salt (sodium chloride) to the
amount of two per cent, causes Amoeba to withdraw its
pseudopods and undergo a Certain amount of shrinkage : it
is then said to pass into a condition of dry-rigor. Under
these circumstances it may be restored to its normal con-
dition by adding a sufficient proportion of water to bring
back the fluid to its original density.
In this connection it is interesting to notice that the dele-
22
AMCEBA
LESS. I
terious effects of an excess of salt are produced only when
the salt is added suddenly. By the very gradual addition of
sodium chloride Amoebae have been brought to live in a four
per cent, solution, one twice as strong as would, if added
suddenly, produce dry-rigor.
From what has been said above on the subject of respira-
tion (p. 17) it follows that free oxygen is necessary for the
existence of Amoeba. Light, on the other hand, appears to
be unnecessary, amoeboid movements having been shown to
go on actively in darkness.
LESSON II
HvEMA'l'OCOCCUS
The rain-water which collects in puddles, open gutters,
&c., is frequently found to have a green or red colour.
The colour is due to the presence of various organisms
— plants or animals — one of the commonest of which is
called Jfcematococcus (or as it is sometimes called Sp/iccrella
or Protococcus) pluvialis.
Like Amoeba, Htematococcus is so small as to require a
high power for its examination. Magnified three o^ four
hundred diameters it has the appearance (Fig. 3, a) of an
ovoidal body, somewhat pointed at one end, and of a bright
green colour, more or less flecked with equally bright red.
Like Amoeba, moreover, it is in constant movement, but
the character of the movement is very different in the two
cases. An active Haematococcus is seen to swim about
the field of the microscope in all directions and with
considerable apparent rapidity. We say apparent rapidity
because the late of progression is magnified to the same
extent as the organism itself, and what appears a racing
speed under the microscope is actually a very slow crawl
when divided by 300. It has been found that such
organisms as Hsematococcus travel at the rate of one foot
in from a quarter of an hour to an hour : or, to express
24
nvI'MATOCOCCUS
LESS.
the fact in another and fairer way, that they travel a distance
equal to two and a half times their own diameter in one
second. In swimming the i)ointed end is always directed
Fig. 3 . — A. Hicntatoioccus motile phase. specimen,
shovvini^ proto])lasm with cliiomalopliore (c///-) and pyrenuids (/J'r),
cell-wall ((. ic) eonneettd cell-hody l)y protoplasmic filaments, and
flagella {//.). I'lie st ale to the left applies to Kigs. A — D.
B. Resting stage of the same, showing mu'Icus (;///) with “ nucleolus”
(;///'), and thick cell-wall (t. 7a) in contact witli protoplasm.
C, The same, showing division of the tell-hody in the resting stage
into four daughtei -cells,
i>. The same, showing the (h'velojunent of flagella and detached cell-
wall by the ilaughter-eells before their liberation from the enclosing
mother-eel l-walk
K. 1 lit )nati\oii lainslri'i^ showing nucleus [uit), single large
pyienoid (/'iv'), and contractile vacuole (<. rai).
F. Diagram illustrating the mo\ement of a tlagellum : ah, its base ;
c", different positions vassumed b} its apex, (e, after Rutschli. )
FLAGELLA
25
forwards and the forward movement is accompanied by a
rotation of the organism upon its longer axis.
Careful watching shows that the outline of a swimming
Haematococcus does not change, so that there is evidently
no protrusion of pseudopods, and at first the cause of
the movement appears rather mysterious. Sooner or later,
however, the little creature is sure to come to rest, and there
can then be seen projecting from the pointed end two exces-
sively delicate colourless threads (Fig. 3, A,y^), each about
half as long again as the animalcule itself : these are called
flagella or sometimes cilia.^ In a Lloematococcus which
has come to rest these can often be seen gently waving
from side to side : when this .slow movement is exchanged
for a rapid one the whole organism is propelled through
the water, the flagella acting like a pair of extremely fine
and flexible fins or paddles. Thus the movement of
Hsematococcus is not amceboid, j^roduced by the pro-
trusion and withdrav^al of pseudopods, but is ciliary^ i.e,^
due to the rapid vibration of cilia or flagella.
The flagella are still more clearly seen by adding a drop
of iodine solution to the water : this immediately kills and
stains the organism, and the flagella are seen to take on a
distinct yellow tint. By this and other tests it is shown that
Hsematococcus, like Amoeba, consists of protoplasm, and
that the flagella are simply filamentous processes of the
protoplasm.
It was mentioned above that in swimming the pointed end
^ The word cilium is sometimes used as a general term to include
any delicate vibratile process of protoplasm : often, however, it is used
in a restricted sense for a rhythmically vibrating thread, of which each
cell bears a considerable number (see Fig. 8 , E, and Fig. 20) ; a flagel-
lum is a cilium having a whip-lash-like movement, each cell bearing
only a limited number — one or two, or occasionally as many as four.
26
h^:matococcus
LESS.
with the flagella goes first ; this may therefore be distin-
guished as the anterior extremity, the oj)posite or blunt
end being posterior. So that as compared with Aniceba,
Haematococcus exhibits a differentiation of structure: an
anterior and a posterior end can be distinguished, and a
part of the protoplasm is diflerentiated or set apart as
flagella.
The green colour of the body is due to the presence of
a special pigment called chlorophyll^ the substance to which
the colour of leaves is due. That this is something cpiite
distinct from the protoplasm may be seen by treatment with
alcohol, which simply kills and coagulates the protoplasm,
but completely dissolves out the chlorophyll, producing a
clear green solution. The solution, although green by trans-
mitted light, is red under a strong reflected light, and is
hence fluorescent: when examined through the spectro-
scope ^’t has the effect of absorbing the whole of the blue
and violet end of the spectrum as well as a part of the red.
The red colour which occurs in so many individuals, some-
times entirely replacing the green, is due to a colouring
matter closely alKed in its properties to chlorophyll and
called hcematochrome.
At first sight the chlorophyll appears to be evenly distri-
buted over the whole body, but accurate examination under
a high power shows it to be lodged in a structure called a
chromatophore (Fig. 3, a , chi^y which forms a layer immedi-
ately beneath the surface, and in this case is relatively large
and urn-shaped. It consists of a protoplasmic substance
impregnated with chlorophyll.
After solution of the chlorophyll with alcohol a nucleus
( b , nu.) can be made out ; like the nucleus of Amceba it is
stained by iodine, magenta, &c. Other bodies which might
easily be mistaken for nuclei'^are also visible in the living
II
CELL-WALL
27
organism. These are small ovoidal structures ( a , pyr^^
with clearly defined outlines occurring in varying numbers
in the chromatophores. When treated with iodine they
assume a deep, apparently black but really dark blue,
colour. The assumption of a blue colour with iodine is the
characteristic test of the well-known substance starchy as
can be seen by letting a few drops of a weak solution of
iodine fall upon some ordinary washing starch. The bodies
in question have been found to consist of a proteid substance
covered with a layer of starch, and arc called pyrenoids.
Starch itself is a definite chemical compound belonging
to the group of carbohydrates^ />., bodies containing the
elements carbon, hydrogen, and oxygen : its formula is
C«H,oO,.
In Hjematococcus pluvialis there is usually said to be no
contractile vacuole, but in another species, PI. lacustris, this
structure is present as a minute space near the anterior or
pointed end (Fig. 3, e , c. vac).
There is still another characteristic structure to which no
reference has yet been made. This appears at the first view
something like a delicate haze around the Ted or green body,
but by careful focusing is seen to be really an extremely thin
globular shell ( a , c.w.) composed of some colourless trans-
parent material and separated, by a sj^ace containing water,
from the body, to which it is connected by very delicate
radiating strands of protoplasm. It is perforated by two
extremely minute apertures for the passage of the flagella.
Obviously we may consider this shell as a cyst or ce//-
wall differing from that of an encysted Am(jeba (Fig. i, d)
in not being in close contact with the protoplasm.
A more important difference, however, lies in its chemical
composition. The cyst or cell-wall of Amoeba, as stated in
the preceding lesson (p. ii) is very probably nitrogenous :
28
H^MATOCOCCUS
LESS.
that of Hjaemalococcus, on the other hand, is formed of a
carbohydrate called cellulose, allied in composition to
starch, sugar, and gum, and having the formula O5.
Many vegetable substances, such as cotton, consist of
cellulose, and wood is a modification of the same com-
pound. Cellulose is stained yellow by iodine, but iodine
and sulphuric acid together turn it blue, and a similar
colour is produced by a solution of iodine and potassium
iodide in zinc chloride known as Schulze’s solution. These
tests are quite easily applied to Hjematococcus : the proto-
plasm stains a deep yellowish-brown, and around it is seen
a sort of blue cloud due to the stained and partly-dissolved
cell-wall.
It has been stated that in stagnant water in which it has
been cultivated for a length of time Hsematococcus some-
times assumes an amoeboid form. In any case, after leading
an active existence for a longer or shorter time it comes to
rest, loses its flagella, and throws around itself a thick cell-
wall of cellulose (Fig. 3, b), thus becoming encysted. So
that, as in Amoeba, there is an alternation of an active
or motile with a stationary or resting condition.
In the matter of nutrition the differences between Haema-
tococcus and Amoeba are very marked and indeed funda-
mental. As we have seen, Haematococcus has no pseudopods,
and therefore cannot take in solid food after the manner
of Amoeba : moreover, even in its active condition it is
usually surrounded by an imperforate cell-wall, which of
course quite precludes the possibility of ingestion. As a
matter of observation, also, however long it is watched it is
never seen to feed in the ordinary sense of the word. Never-
theless it must take in food in some way or other, or the de-
composition of its protoplasm would soon bring it to an end.
II
DECOMPOSITION OF CARBON DIOXIDE
29
Hsematococcus lives in rain-water. This is never pure
water, but always contains certain mineral salts in solution,
especially nitrates, ammonia salts, and often sodium chloride
or common table salt. These salts, being crystalloids, can
and do diffuse into the water of organization of the ani-
malcule, so that we may consider its protoplasm to be con-
stantly permeated by a very weak saline solution, the most
important elements contained in which are oxygen, hydro-
gen, nitrogen, potassium, sodium, calcium, sulphur, and
phosphorus. It must be remarked, however, that the
diffusion of these salts does not take place in the same uni-
form manner as it would through parchment or other dead
membrane. The living protoplasm has the power of
determining the extent to which each constituent of the
solution shall be absorbed.
If water containing a large quantity of Haematococcus
is exposed to sunlight, minute bubbles are found to appear
in it, and these bubbles, if collected and properly tested,
are found to consist largely of oxygen. Accurate chemical
analysis has shown that this oxygen is produced by the de-
composition of the carbon dioxide contained in solution in
rain-water, and indeed in all water exposed to the air, the
gas, which is always present in small quantities in the
atmosphere, being very soluble in vrater.
As the carbon dioxide is decomposed in this way, its
oxygen being given off, it is evident that its carbon must be
retained. As a matter of fact it is retained by the organism
but not in the form of carbon ; in all probability a double
decomposition takes place between the carbon dioxide ab-
sorbed and the water of organization, the result being the
liberation of oxygen in the form of gas and the simultaneous
production of some extremely .simple form of carbohydrate,
t\e. some compound of carbon, hydrogen, and oxygen,
30 H/EMATOCOCCUS less.
with a comparatively small number of atoms to the
molecule.
The next stej) seems to be that the carbohydrate thus
formed unites with the ammonia salts or the nitrates absorbed
from the surrounding water, the result being the formation
of some comparatively simple nitrogenous compound, prob-
ably belonging to the class of amides, one of the best
known of which - asparagin — has the formula QiHgN203.
Then further combinations take place, substances of greater
and greater complexity are produced, sulphur from the ab-
sorbed sulphates enters into combination, and protcids are
formed. From these, finally, fresh living protoplasm
arises.
From the foregoing account, which only aims at giving
the very briefest outline of a subject as yet imperfectly un-
derstood, it will be seen that, as in Ama'ba, the final result
of the nutritive process is the manufacture of protoplasm,
and that this result is attained by the formation of various
substances of increasing comjficxity or anastates (see p. i8).
But it must be noted that the steps in this process of con-
structive metabolism are widely different in the two cases.
In Amoeba we start with living protoplasm — that of the prey
— which is killed and broken up into diffusible proteids,
these being afterwards rc-combined to form new molecules
of the living i)rotoplasm of Amoeba. So that the food of
Amoeba is, to begin with, as complex as itself, and is first
broken down by digestion into simpler compounds, these
being afterwards re-combined into more complex ones. In
Hsematococcus, on the other hand, w^e start with extremely
simple compounds, such as carbon dioxide, water, nitrates,
sulphates, &c. Nothing which can be properly called diges-
tion, a breaking up and dissolving of the food, takes
place, but its various constituents are combined into sub-
n NUTklTION 31
stances of gradually increasing complexity, protoplasm, as
before, being the final result.
To express the matter in another way : Amoeba can only
make protoplasm out of proteids already formed by some
other organism : Haematococciis can form it out of simple
liquid and gaseous inorganic materials.
Speaking generally, it may be said that these two methods
of nutrition arc respectively characteristic of the two great
groups of living things. Animals require solid food con-
taining ready-made proteids, and cannot build uj) their pro-
toplasm out of simpler compounds. Green plants, />., all
the ordinary trees, .shrubs, weeds, &c., take only liquid and
gaseous food, and build up their protoplasm out of carbon
dioxide, water, and mineral .salts. The first of these methods
of nutrition is conveniently distinguished as holozoic^ or
wholly-animal, the second as holophytic^ or wholly-vegetal.
It is important to note that only those plants or parts of
plants in which chlorophyll is present are capable of holo-
phytic nutrition. Whatever may be the precise way in which
the process is effected, it is certain that the dccomj)osition
of carbon dioxide which characterizes this form of nutrition
is a function of chlorophyll, or to speak more accurately, of
chromatophores, since there is reason for thinking that
it is the protoplasm of these bodies and not the actual green
pigment which is the active agent in the process.
Moreover, it must not be forgotten that the decomposition
of carbon dioxide is carried on only during daylight, so that
organisms in which holophytic nutrition obtains are depend-
ent upon the sun for their very existence. While Amoeba
derives its energy from the breaking down of the proteids
in its food (see p. 15), the food of Haematococcus is too
simple to serve as a source of energy, and it is only by the
help of sunlight that the work of constructive metabolism
ll/KMATOCOCCUS
tESS.
3 ^
can be carried on. This may be expressed by saying that
Hsematococciis, in common with other organisms contain-
ing chlorophyll, is supplied with kinetic energy (in the form
of light or radiant energy) directly by the sun.
As in Amoeba, destructive metabolism is constantly going
on, side by side with constructive. The protoplasm becomes
oxidized, water, carbon dioxide, and nitrogenous waste
matters being formed and finally got rid of. Obviously
then, absorption of oxygen must take jilace, or in other
words, respiration must be one of the functions of the pro-
toplasm of Hrematococcus as of that of Amoeba. In many
green, />., chlorophyll -containing, jilants, this has been proved
to be the case ; respiration, the taking in of oxygen and
giving out of carbon dioxide, is constantly going on, but
during daylight is obscured by the converse process— the
taking in of carbon dioxide for nutritive i)urposes and the
giving out of the oxygen liberated by its decomiiosition. In
darkness, when this latter process is in abeyance, the
occurrence of resjiiration is more readily ascertained.
Owing to the constant decomposition, during sunlight, of
carbon dioxide, a larger volume of oxygen than of carbon
dioxide is evolved ; and if an analysis were made of all
the ingesta of the organism (carbon dioxide mineral
salts />/hs respiratory oxygen) they would be found to con-
tain less oxygen than the egesta (oxygen from decomjiosition
of carbon dioxide J>/us water, excreted cm*bon dioxide and
nitrogenous waste) ; so that the nutritive process in Hsema-
tococcus is, as a whole, a process of deoxidation. In
Amoeba, on the other hand, the ingesta (food plus respi-
ratory oxygen) contain more oxygen than the egesta (faeces
plus carbon dioxide, water, and nitrogenous excreta), the
nutritive process being therefore on the whole one of
oxidation. This difference is, speaking broadly, character-
II
CILIARY MOVEMENT
33
istic of plants and animals generally ; animals, as a rule,
take in more free oxygen than they give out, while green
plants always give out more than they take in.
But destructive metabolism is manifested not only in the
formation of waste products, but in that of substances,
simpler than protoi)lasm, which remain an integral part of
the organism, viz., cellulose and starch. The cell-wall is
probably formed by the conversion of a thin superlicial
layer of protoplasm into cellulose, the cyst attaining its final
thickness by frequent repetition of the process (see ]>. 14).
The starch of the pyrenoids is apparently formed by a similar
process of decomposition or destructive metabolism of pro-
toplasm, growth taking place, in both instances, by accretion
and not by intussusception.
We see then that destructive metabolism may result in the
formation of (a) waste products and {b) plastic products^
the former being got rid of as of no further use, while
the latter remain an integral part of the organism.
Let us now turn once more to the movements of Hremato-
coccus, and consider in some detail the manner of their
performance.
Each flagellum (Fig. 3, is a thread of protoplasm of
uniform diameter except at its distal or free end where it
tapers to a point. The lashing movements are brought
about by the flagellum bending successively in different
directions ; for instance, if in Fig. 3 f, abc represents it in
the position of rest, abc will show the form assumed when
it is deflected to the left, and abc" when the bending is
towards the right. In the position abc the two sides 0^, ac
are obviously equal to one another, but in the flexed
positions it is equally obvious that the concave sides ac\ be"
are shorter than the convex sides bc\ ac" \ in other words, as
i>
34
II/KMATOCOCCUS
LESS.
the flagellum bends to the left side ac becomes shortened,
as it bends to the right the side he.
This may be otherwise expressed by saying that in bend-
ing to the left the side ac contracts (see p. lo), in bending
to the right the side bc^ or that the movement is performed
by the alternate contraction of opposite sides of the
flagellum.
'I'hus the ciliary movement of Hcematococcus, like the
amcjcboid movement of Amoeba, is a phenomenon of cm-
inutility. Imagine an Amoeba to draw in all its pseudo-
pods but two, and to j)rotrude these two until they became
mere threads ; imagine further these threads to contract
regularly and rapidly instead of irregularly and slowly ; the
result would be the substitution of pseudopods by flagella,
/>,, of temporary slow-moving processes of i)rotoplasm by
permanent ra\)idly-moving ones.
'lo put the matter in another way : in Amoeba the
function of contractility is performed by the whole organism ;
in Hoematococcus it is discharged by a small part only, viz.,
the flagella, the rest of the protoplasm being incapable of
movement. We have therefore in Hsematococcus a dif-
ferentiation of structure accompanied by a differentiation of
function or divisiori of physiological labour.
The expression “division of physiological labour” was
invented by the great French physiologist, Henri Milne-
Edwards, to express the fact that a sort of rough correspond-
ence exists between lowly and highly organized animals
and plants on the one hand, and lowly and highly organized
human societies on tlie other. In primitive communities
there is little or no division of labour : every man is his
own butcher, baker, soldier, doctor, &c., there is no distinc-
tion between “classes” and “masses,” and each individual
is to a great extent independent of all the rest. Whereas in
II
DIMORPHISM
35
com])lex civilized communities society is differentiated into
politicians, soldiers, professional men, mechanics, labourers,
and so on, each class being to a great extent dependent on
every other. This comparison of an advanced society with
a high organism is at least as old as /Esop, who gives
expression to it in the well-known fable of “ the Belly and
Members.”
We see the very first step towards a division of labour in
the minute organism now under consideration. If we could
cut off a pseudopod of Amceba the creature would be little
or none the worse, since every part would be capable of
sending off similar processes, and so movement would be in
no way hindered. But if we could amputate the flagella of
Hrematococcus its movements would be absolutely stopped.
Hrematococcus multiplies only in the resting condition
(p. 28, and Fig. 3, n) ; as in Amoeba its protoplasm undergoes
simple or binary fission, but with the peculiarity that the
process is immediately repeated, so that four daughier’CeHs
are produced within the single mothcr-ccll-wall (Fig. 3 c).
By the rupture of the latter the daughter-cells are set
free in the ordinary motile form ; sometimes they acquire
their flagella and detached cell-wall before making their
escape (d).
Under certain circumstances the resting form divides into
eight or even more daughter-cells, and these when liberated
are found to be smaller than the ordinary motile form, and
to have no cell-wall. Hoematococcus is therefore dimorphic^
i.e,y occurs, in the motile condition, under two distinct
forms : the larger or ordinary form with detached cell- wall
is called a megazooidy the smaller form without a cell-wall a
microzooid.
LESSON III
HETEROMITA
When animal or vegetable matter is placed in water and
allowed to stand at the ordinary temperature, the well known
process called decomposition sooner or later sets in, the
water becoming turbid and acquiring a bad smell. A drop
of it examined under the microscope is then found to teem
with minute organisms. To one of these, called “the
Springing Monad,” or in the language of zoology, Hetero-
mita rostratUy we must now direct our attention; it is
found in infusion of cod^s head which has been allowed to
stand for two or three months.
Heteromita (Fig. 4, a) is considerably smaller than either
Amceba or Hoematococcus, being only isihjs inch)
in average length. It has a certain resemblance in general
form to Hsematococcus, being somewhat ovoidal and pointed
at one end. Like Haematococcus also it has two flagella,
but only one of these (jfl i) proceeds from its beak-like
anterior end and is directed forwards as the creature swims ;
the other {fl, 2) springs a short distance from the beak, and
in the ordinary swimming position is trailed after the
organism as in a^ and Thus in Heteromita, besides an
anterior and a posterior end, we may distinguish a ventral
LESS. Ill
NUTRITION
37
surface which is directed downwards in the ordinary
position, and bears the second or trailing flagellum, and an
opposite or dorsal surface directed upwards.
Often instead of swimming freely in the fluid a Hetero-
mita is found anchored as it were to a bit of the decompos-
ing substance by its ventral flagellum as in Under
these circumstances it is in constant movement, springing
backwards and forwards by alternately coiling and uncoiling
the attached ventral flagellum. The general character of
the mavement will be readily understood from the figure, in
which shows the monad with coiled flagellum, a^ after it
has sprung forward to the full extent of the flagellum. It
is from this curious habit that the name ‘‘ springing monad ”
is derived.
Towards the posterior end of the body is a nucleus {nu\
and at the anterior end a contractile vacuole (r. vac). There
is no trace of an investing membrane or cell-wall, and the
protoplasm is colourless. Also, as is invariably the case
with organisms devoid of chlorophyll, there is no starch.
In considering the nutrition of Ueteromita it is ncces.sary,
first of all, to take into consideration the precise nature of
its surroundings. It lives, as already stated, in decomposing
infusions of animal matter. Such infusions contain proteids
in solution, in part split up by the process of decomposition
into simpler compounds some of which are diffusible ; this
process is due, as we shall see hereafter (Lesson VIII.), to
the action of the minute organisms known as Bacteria,
which are always present in vast numbers in putrescent
substances.
As Heteromita contains no chlorophyll its nutrition is
obviously' not holophytic. Observation seems to show
pretty conclusively that it is not holozoic ; apart from the
LESS. Ill
NUTRITION
39
{c. vai\ anterior flagellum {Ji, i), and coiled ventral flagellum (//. 2)
by which the organism is anchored ; a'-* shows the position at the
forward limit of the spring, the ventral flagellum being fully extended.
three stages in the longitudinal fission of the anchored form.
Three stages in the transverse fission of the same ; Jl.
rudiment of newly formed anterior flagellum.
1 ) 1 — 1)3^ three stages in the fission of the free-swimming form : Jl. 2 ^,
rudiment of the newIy*foimed ventral flagella.
free-swimming and ancliored forms about to conjugate ; E*-*, com-
mencement of conjugation : E-\ E’*, two stages in the development of
the zygote : the fully formed zygote : E*’, dehiscence of the zygote
and emission of spores.
F^—F^ four stages in the development of the spores.
(After Dallinger.)
fact that it possesses neither mouth nor pscudopods, examples
have been kept under observation for hours together by
trained microscopists, and have never been oliserved to
ingest the bacteria or other particles, dead or alive, contained
in the fluid. There remains only one way in which
nutrition can take place, namely, by absorption of the
proteids and other nutrient substances in the solution,
by these substances diffusing into the water of organisation
of the monad. Whether the proteids arc rendered diffusible
by the process of decomposition alone, />., by the action
of bacteria (see p. 91), or whether a kind of surface
digestion takes jilacc, the protoplasm of Heteromita con-
verting the proteids in immediate contact with it into pep-
tones or allied compounds, is not certain.
Thus Heteromita feeds neither by taking solid pro-
teinaceous food into its interior (holozoic nutrition) nor by
decomposing carbon dioxide and combining the carbon with
water and mineral .salts (holophytic nutrition), but by absorb-
ing decomposing proteids and other nutrient substances in
the liquid form ; this is the saprophytic mode of nutrition .
It will be seen that the main difference between saprophytic
and holozoic nutrition is that in the former digestion,
the process of rendering food-stuffs soluble and diffusible,
40
HETEROMITA
LESS.
takes place outside the body so that constructive meta-
bolism can begin at once.
It is worthy of notice that while the process of feeding is
strictly intermittent in Amoeba, which takes in food at ir-
regular intervals, and largely intermittent in Hsematococcus,
in which the decomposition of carbon dioxide takes place only
during daylight, in Heteromita it is continuous, the organism
living in a solution of putrefying proteids which it is con-
stantly absorbing. It may be said to live immersed in an
immense cauldron of broth which it is for ever imbibing,
not by its mouth, for it has none, but by the whole surface
of its body.
Respiration and excretion probably take place in the same
manner as in Amoeba. It has been shown that the optimum
temperature for saprophytic monads is about i8° C., the
ultra-maxinuim or thermal death-point about 6o® C. But
it is an interesting fact that by very slowly increasing the
tcmi)eraturc. Dr. Dallinger was able in the course of several
months to accustom some of these forms — not Heteromita
itself but closely allied genera— to live at a temperature
exceeding 68® C.
The ordinary method of reproduction is by simple fission,
the process affecting not only the body but the flagella
as well. In Fig. 4, the commencement of fission is
shown ; the anterior flagellum has undergone complete
longitudinal division, while the split has extended only about
a third of the length of the body and ventral flagellum. In
B- the process has gone further, and in the products of
division are on the point of separating.
More frequently, however, fission instead of being longitudinal, i.e.y
in the direction of the long axis of the monad, is transverse, i.e.^ at
right angles to the long axis. This process is shown in c* — c*, and is
seen to differ from that described in the preceding paragraph in the cir-
Ill
CONJUGATION
41
cumstance that the anterior flagellum of the parent form is unaffected, and
becomes without alteration the anterior flagellum of one of the daughter-
forms — that to the right in the figures. The anterior flagellum of the
other product of division — that to the left — is a new structure formed as
an outgrowth from the body : its commencement is shown in c’,yf. i'.
These two modes of fission — longitudinal and transverse — both occur
in the anchored form of Heteromita, i.e.^ in individuals attached by
the ventral flagellum. The free-swimming form presents a third
variety of the process. It comes to rest, loses its regular outline (i)’)
becoming almost amceboid in form and finally (d'*^) globular. Division
then takes place : the flagella of the parent become each the anterior
flagellum of one of the daughter-cells (compare d'**, and D®), while
their ventral flagella are formed by the splitting of a little outgrowth of
the dividing body (d'-*, fl, 2').
As in Amoeba fission is invariably preceded by division of
the nucleus.
But in Heteromita fission is not the only mode of repro-
duction. Under certain circumstances a frce-.s>vimming form
apj)roaches an anchored form, and applies itself to it in such
a way that the posterior ends of the two are in contact (ic^).
The two individuals then fuse with one another as completely
as two drops of gum on a plate unite when brought into
contact. Fusion of the nuclei also takes place, and there is
formed an irregular body (e^) with a single nucleus and
with two flagella at each end. This swims about freely, and
as it does so the last trace of distinction between the two
monads of which it is formed is lost, and a triangular form is
assumed (e^), the two pairs of flagella being situated at two
of the angles. Still later the protoplasm of this triangular
body loses all trace of nucleus, granules, &c., and becomes
perfectly clear (e**) : then it comes to rest and loses its
flagella, appearing as a clear, homogeneous, three-cornered
sac with slightly convex sides (e^). This body, formed by
the conjugation of the two monads, is called a zygote^ the
two conjugating individuals being distinguished as gametes.
42
HETEROMITA
LESS.
The zygote remains quiescent for some time, and then,
after undergoing wave-like movements of its surface, bursts
at its ‘three angles its contents escaping in the form of
granules called spores^ so minute as to be barely visible even
under the highest powers of the best modern microscopes.
They are formed by the protoplasm of the zygote dividing
into an immense number of separate masses, a process known
as tnultiple fission.
Carefully watched, these almost ultra-microscopic particles
(f^) are found to grow into clear visibility and to take on a
distinctly oval shape Still increasing in size they
develop a ventral flagellum (f*^) which is at first quite
quiescent : finally, the pointed end sends out a process which
becomes an anterior flagellum (f*^). The spore has now
become a Heteromita resembling the parent form in all but
size. As growth proceeds a nucleus becomes apparent.
All analogy leads us to believe that this is not a new
structure, but that the multiple fission of the protoplasm of
the zygote is preceded by the multiple fission of its nucleus,
each spore having thus its own ultra-microscopic nucleus
from the very first.
It will be seen that this mode of multiplication following
conjugation differs from ordinary multiplication by fission in
that it requires the co-operation of two individuals which
undergo complete fusion. As we shall sec more plainly
later on (Lessons XV., XVI.) conjugation leads to the
simplest case of sexual reproduction^ differing from the sexual
reproduction of the higher organisms in that the two conjugat-
ing bodies or gametes are each an entire individual, and in the
further circumstance that the gametes resemble one another
in form and size, so that there is no distinction of sex,^ but
each takes an equal and similar share in the production of
^ It might perhaps be allowable to consider the active, free-swimming
Ill
LIFE HISTORY
43
the zygote. Binary fission, on the other hand, is an example
of asexual reproduction.
Notice also another important fact. The spores when
first emitted from the ruptured zygote are mere granules of
protoplasm, approaching as nearly as anything in nature to
the mathematical definition of a point, ‘‘ without parts and
without magnitude.” And, during its growth, a s])ore in-
creases not only in size but also in complexity, in other
words undergoes a progressive differentiation or development.
This is an instance of the principle known as Von Baer’s
law, according to which ‘’development is a progress from
the simple to the complex, from the general to the special,
from the homogeneous to the heterogeneous.” In Heteromita,
then, we have our first instance of development, since in
simple fission there is no development, each i)roduct of
division being, from the first, similar to the parent in all but
size.
Lastly, Heteromita is the first instance we have had of
an organism wMth a definite life-history. It multiplies
asexually by simple fission, producing free-swimming and
anchored forms : these conjugate in pairs forming a zygote,
in which, by multiple fission, numerous spores are formed :
the spores develop into the adult form, asexual multiplica-
tion begins once more, and so the cycle of existence is
completed.
It must be borne in mind that further researches may
reveal the occurrence of a true sexual process in Amoeba
and Haematococcus pluvialis : in other species of Haemato-
coccus conjugation of the microzooids is known to take
place.
monad which seeks and attaches itself to the anchored form as a male,
and the passive anchored form as a female gamete (see Lesson XII. ). '
LESSON IV
EUGLENA
The rain-water collected in puddles by the road-side, on
roofs, &c., is often found to have a bright green colour:
this is sometimes due to the presence of delicate water
weeds visible to the naked eye (Lessqns XVI. and XIX.), but
frequently the water when held up to the light in a glass
vessel appears uniformly green, no suspended matter being
visible to the unaided sight. Under these circumstances
the green colour is frequently due to the presence of vast
numbers of an organism known as Euglena viridis.
Although microscopic, Euglenais considerably larger than
either Hjematococcus or Heteromita, its length varying from
, mm. to I mm. The body is spindle-shaped, wide in the
middle and narrow at both ends (Fig. s, a— -e) ; one
extremity is blunter than the other, and from it proceeds
a single , long flagellum (fl) by the action of which the
organism swims with great rapidity, the flagellum being,
as in Haematococcus, directed forwards. Besides its rapid
swimming movements Euglena frequently performs slow
movements of contraction and expansion, something like
those of a short worm, the body becoming broadened out
first at the anterior end, then in the middle, then at the
LESS. I\
(;eneral 1 iiAKAnr.R^
45
posterior end, twisting to the right and Icfl, and so on (Fig,
5, A— [)). These nio\enients are so (haraeterislie o| the
genus that the name is applied to them.
' I
ii
Fig. $.~Eu^/ma mrtdis.
A—l), four views of the living organism, showing the changes of form
produced by the characteristic cuglenoid inoveinenis.
E, enlarged view, showing the nucleus {;///), reservoir of tin; eon-
tactile vacuole (o rue), with adjoining pigment spot, and gullet with a
single tlagellum springing from it.
F, enlarged view of the anterior end of v, showing ]>igincnl-spf)t
(/|g) and reservoir (o vac), mouth (w), gullet (tn), and origin of
flagellum (/).
G, resting form after binary fission, showing cyst or cell-wall {cy),
and the nuclei {mi) and reservoirs {c. vac) of the daughter-cells.
II, active form showing contractile vacuole (r. vac), reservoir (?'), and
paramylum-bodies {f).
(a -c, after Saullc Kent • II, from Butschli after Klehs.)
The body consists of protoplasm covered with a very
delicate skin or cuticle which is often finely striated, and
is to be looked upon as a su[)erficial hardening of the
protoplasm. The green colour is due to the prc.sence of
46
EUGLENA
LESS.
chloroi)]iyll, which tinges all the central part body,
the two ends being colourless. It is difficult^BPake out
whether the chlorophyll is lodged in one chroi^^Bhore or
in several.
In Haematococcus we saw that chloropl|pWas asso-
ciated with starch (p. 27). In Euglena there are, near the
middle of the body, a number of grains of paramylum
(h,/), a carbohydrate of the same composition as starch
(CgH^QOfi), but differing from it in remaining uncoloured
by iodine.
Water containing Euglena gives off bubbles of oxygen in
sunlight : as in Haematococcus the carbon dioxide in solution
in the water is decomposed in the presence of chlorophyll,
its oxygen evolved, and its carbon combined with the
elements of water and used in nutrition. For a long time
Euglena was thought to be nourished entirely in this way,
but there is a good deal of reason for thinking that this is
not the case.
When the anterior end of a Euglena is very highly
magnified it is found to have the form shown in Fig. 5, f.
It is produced into a blunt snout-like extremity at the base
of which is a conical depression (a*s) leading into the soft
internal protoplasm : — just the sort of depression one could
make in a clay model of Euglena by thrusting one’s finger or
the end of a pencil into the clay. From the bottom of this
tube the flagellum arises, and by its continual movement
gives rise to a sort of whirlpool in the neighbourhood. By
the current thus produced minute solid food-particles are
swept down the tube and forced into the soft internal
protoplasm, where they doubtless become digested in the
same way as the substances ingested by an Amoeba. That
solid particles are so ingested by Euglena has been proved
by diffusing finely powdered carmine in the water, when the
IV
MOUTH AND GULLET
47
coloured^ "J^ticles were seen to be swallowed in the way
described. .
The dep^ssion in question is therefore a gu//efy and its
external aperture or margin (m) is a mout/i, Euglena,
like Amoeba, takes in solid food, but instead of ingesting it
at almost any part of the body, it can do so only at one
particular point where there is a special ingeslive aperture
or* mouth. This is clearly a case of specialization or
differentiation of structure : in virtue of the possession of a
mouth and gullet Euglena is more highly organized than
Amoeba.
It thus appears that in Euglena nutrition is both holozoic
and holophytic ; very probably it is mainly holophytic during
daylight and holozoic in darkness.
Near the centre of the body or somewhat towards the
posterior end is a well-marked globular nucleus (e, m), and
at the anterior end is a clear space (c, me) looking very like
a contractile vacuole. It has been shown, however, that
this space is in reality a non-contractile 'cavity or reservoir
(h, r) into which the true contractile vacuole {c. vac) opens,
and which itself discharges into the gullet.
In close relation with the reservoir is found a little bright
red speck (p^ called the pigfnent spot or stigma. It con-
sists of hsematochrome (see p. 26) and is curiously like an
eye in appearance, so much so that it is often known as the
eye-spot. Recent experiments seem to show that it is
specially sensitive to light and is therefore a true eye in the
sense of a light-perceiving organ although having no actual
visual function.
As in Haematococcus a resting condition alternates with
the motile phase: the organism loses its flagellum and
48
EUGLENA
LESS. IV
surrounds itseif with a cyst of cellulose (Fig. 5, g, (y), from
which, after a period of rest, it emerges to resume active
life.
Reproduction takes place by simple fission of the resting
form, the plane of division being always longitudinal (g).
Sometimes each product of division or daughter-cell divides
again : finally the two, or four, or sometimes even eight
daughter-cells emerge from the cyst as active Euglenae.
A process of multiple fission (p. 42) has also been de-
scribed, numerous minute active spores being produced
which gradually assume the ordinary form and size.
LESSON V
PROTOMYXA AND THE MYCETOZOA
When Professor Haeckel was investigating the zoology of
the Canary Islands more than twenty years ago he discovered
a very remarkable organism which he named Protomyxa
aurantiaca. It was found in sea- water attached to a shell
called Spirula^ and was at once noticeable from the bright
orange colour which suggested its specific name. Appar-
ently no one has since been fortunate enough to find it.
In its fully developed stage Protomyxa is the largest of all
the organisms we have yet studied, being fully i mm. inch)
in diameter, and therefore v isible to the naked eye as a
small orange speck . In general appearance (Fig. 6, a), it is
not unlike an immense Amceba, the chief difference lying
in the fact that the pseudopods {psd) instead of being short,
blunt processes, few in number (comp. Fig. i, p. 2 ) are very
numerous, slender, branching threads which often unite with
one another so as to form networks. No nucleus was ob-
served ^ and no contractile vacuole, but it is quite^ possible
that a renewed examination might prove the presence of one
or both of these structures.
The figure (a) is enough to show that nutriticto is holozoic;
' See p, 9, note.
£
Fig. 6. — Protoviyxa au7'antiaca.
\. the Hvinsjj oiganisin y>la^>n<>(Hum), sliowing fine branched pseudo-
(/■'</) and >nNciaI ingc-^led oiganisnis.
H, the same, ene\sled : < r the eell-wall
c, llie ]nettiplasm of the cney-ted form breaking up into spores.
1), deliisieme ol die evsi ami emetgenee«)f
F, t1.ic:ellu].e wlneh afteruaids l)econie converted into
G, anux'buLe uniting to form a plasmodium (After Haeckel.)
LESS. V LIFE-HISTORY 51
the specimen has ingested several minute organisms and is
in the act of capturing another.
But the main interest of Protomyxa lies in its very curious
and complicated life-history. After crawling over the Spirula
shell for a longer or shorter time it draws in its pseudopods,
comes to rest, and surrounds itself with a cyst (b, <7). The
composition of the cyst is not known, but it is apparently not
cellulose, since it is not coloured by iodine and sulphuric
acid (p. 28).
Next, the encysted protoplasm undergoes multiple fission,
dividing into a number of spores (c). Soon the cyst bursts
and its contents emerge (o) as bodies which differ utterly in
appearance from the amoeboid form from which w^e started.
Each spore has in fact become a little ovoid body of an
orange colour, provided with a single flagellum (e, fl) by the
lashing of which it swims through the water after the manner
of a monad.
It is convenient to have a name by which to distinguish
these flagellate bodies, just as we have special names for
the young of the higher animals, such as tadpoles or kittens.
From the fact of tlieir distinguishing character being the
possession of a flagellum they are called fla^ellulte ; the
same name will be applied to the fla gellate young of various
other organisms w’hich w'e shall study hereafter.
After swimming about actively for a time each flagellula
settles down on some convenient substratum and undergoes
a remarkable change ; its movements become sluggish, its
outline irregular, and its flagellum short and thick, until it
finally takes on the form of a little Amoeba (f). For this
stage also a name is required: it is not an Amoeba but an
amoeboid phase in the life-history of a totally different
organism : it is called an amcsbula .
The process just described may be taken as a practical
E 2
52
PROTOMYXA AND THK MYCETOZOA
LESS.
proof of the statement made in a previous Lesson (p. 34)
that a flagellum is nothing more than a delicate and rela-
tively permanent pseudopod. In Protomyxa we have a
flagellula directly converted into an amoebula, the flagellum
of the former becoming one of the pseudopods of the
latter.
The amoebuloe thus formed may simply increase in size
and send out numerous delicate pseudopods, thus becoming
converted into the ordinary Protomyxa-form. Frequently,
however, they attain this form by a very curious process :
they come together in twos and threes until they are in
actual contact with one another, when they undergo complete
and permanent fusion (g). In this case the Protomyxa-form
is produced not by the development ot a single amcebula
but by the complete fusion of a variable number of
amoebulje. A body formed in this way^ by^ t^ fus ion of
amcebulce is called a plasmodium ^ so that in the life-history
of Protomyxa we can distinguish an encysted, a ciliated or
flagellate, an amoeboid, and a plasmodial phase.
The nature of a plasmodium will be made clearer by a
brief general consideration of the strange group of organisms
known as Mycetozoa or sometimes slime-fungi, ^* to which
Protomyxa itself very probably belongs. The best known
members of the group occur as gelatinous masses on the
bark of trees, on dead leaves, on the surface of tan-pits, and
sometimes in water. It must be remembered that Mycetozoa
is the name not of a genus, but of a class in which are
included several genera, such as Badhamia^ Physarum, &c.
(see Fig’. 7) : a general account of the class is all that is
necessary for our present purpose.
The Mycetozoa consists of sheets or networks of proto-
plasm which may be as much as 30 cm (i ft.) in diameter,
V
TIIK rLASMODil-M UK lUDllAMIA
S3
Fig. 7. --a, part of the plasmodium of Badhdmia (x 3^) ; a short
psciulopod enclosing a bit of mushroom stem,
n, spore of Phy^arufii.
ihe same, undeigoing dUiiscence.
i>, flagellula- hheratefl fiom sjjores of the same.
E, anKcbiila* formed by metamorphosis of flagellulse,
F, two amrebula? about to fuse : F\ the same after complete union.
G, g', two stages in the formation of a three-celled plasmodiiim.
H, a small plasmodium.
(A, after Lister ; B — H, from Sachs after Cienkowski.)
54
PROTOMYXA AND THE MYCETOZOA
LESS.
and throughout the substance of which are found numerous
nuclei. In this condition they creep about over bark or
some other substance : and in doing so ingest solid food
(Fig. 7, a). It has been proved that they digest protoplasm :
and in one genus pepsin — the constituent of our own gastric
juice by which the digestion of proteids is effected (see j), 12)
— has been found. Theycan also digest starch which has been
swollen by a moderate heat — as in our own bread and rice-
puddings— but are unable to make use of raw starch.
After living in this free condition, like a gigantic terrestrial
Amceba, for a longer or shorter time, either a part or the
whole of the protoplasm becomes encysted ^ and breaks up
into spores. These (b) consist of a globular mass of i)roto-
])lasm covered with a wall of cellulose : the cysts are also
formed of cellulose.
Hy the rupture of the cell-wall of the spore (c) the proto-
plasm is liberated as a flagellula (d) provided with a nucleus
and a contractile vacuole, and frequently exhibiting amoeboid
as well as ciliary movements. After a time the flagellulae
lose their cilia and pass into the condition of amoebulje (e),
which finally fuse to form the plasmodium with which
we started (f— h). In the young plasmodia (o^) the
nuclei of the constituent amoebulce are clearly visible, and
from them the nuclei of the fully developed plasmodia are
probably derived. It would seem, therefore, that in the
fusion of amoebulae to form the plasmodium of Mycetozoa the
cell-bodies (protoplasm) alone coalesce, not the nuclei.
There is a suggestive analogy between this process of
’ The process of formation of the cyst or sporangium is a compH-
catetl one, anti will not be described here. See De Bary, Fungi ^
Mycetozoa^ and Bacteria (Oxford, 1887), and Lister, Catalogue of the
Mycetozoa (London, 1894).
V PLASMODIUM-FORMATION AND CONJUGATION 55
plasmodium-forniation and that of conjugation as seen in
Heteromita. Two Heteromitte fuse and form a z5^gote the
protoplasm of which divides into spores. In Frotomyxa and
the Mycetozoa not two but several amcebtite unite to form
a plasmodium which after a time becomes encysted and
breaks up into spores. So that we might look iii)on the
conjugation of Heteromita as an extremely simi)le plasnio-
dial phase in its life-history, or upon the formation of a
plasmodium by Protomyxa and the Mycetozoa as a process
of multiple conjugation.
There is, however, an important difierence between the
two cases by reason of which the analogy is far from complete.
In Heteromita the nuclei of the two gametes are no longer
visible (p. 41) : they coalesce during conjugation, and
the product of their union subsequently, in all probability,
breaks up to form the nuclei of the spores. In the Myce-
tozoa neither fusion nor ajqmrent disappearance of the
nuclei of the amoebulae has been observed.
LESSON VI
A COMPARISON OF THE FOREGOING ORGANISMS WITH CER-
TAIN CONSTITUENT PARTS OF THE HIGHER ANIMALS
AND PLANTS
When a drop of the blood of a crayfish, lobster, or crab is
examined under a high power, it is found to consist of a
nearly colourless fluid, the plasma^ in which float a number
of minute solid bodies, the hhwd-corpuscks or leucocytes.
Each or these (Fig. 8, a) is a colourless mass of proto-
plasm, reminding one at once of an Amoeba, and on
careful watching the resemblance becomes closer still, for
the corpuscle is seen to put out and withdraw pseudopods
(a 1 — A^) and so gradually to alter its form completely.
Moreover the addition of iodine, logwood, or any other
suitable colouring matter reveals the presence of a large
nucleus (a^, nu) : so that, save for the absence of a con-
tractile vacuole in the leucocyte, the description of Amoeba
in Lesson I. would apply almost equally well to it.
The blood of a fish, a frog (b^), a reptile, or a bird contains
quite similar leucocytes, but in addition there are found in
the blood of these red-blooded animals bodies called red
corpuscles. They are flat oval discs of protoplasm (b^, b®)
Fig. 8. — Typical Animal and VegelaMc (’cIK.
— A^, living leucocyte (blood corpuscle) of a crayfish showing
amo-boid movemenls : A*'’, the same, killed and stained, sliowing
the nucleus («//),
id, leucocyte of the frog, 7iii the nucleus ; B^, two leucocytes
beginning to undergo fusion: B’*, the same after fusion, a l)imKleaf<'
phismodium being formed : id, a leucocyte, undergoing binary fission :
B-’, surface \iew' and B*‘, edge view of a red cor]mscle of tlie same,
;/z/, the nucleus.
c:\ c^, leucocytes of the newt ; in c' particles of vermilion, repre-
sented by black dots, have been ingested.
c^, surface view and edge view of a red corpuscle of man,
p\ columnar epithelial cells from intestine of frog : rd, a similar
EPITHELIAL CELLS
58
cell showing striate<l distal border from which in d’ pseudopods are
protruded.
ciliated epithelial cell from mouth of frog ; e’, e'^ similar cells
from windpipe of dog.
F*, parenchyma cell from root of lily, showing nucleus (;///), vacuoles
(vac), and cell-wall : F-, a similar cell from leaf of bean, showing
nucleus, vacuoles, cell-wall and chromatojihores ((Ar),
(h, id, and E*, after Howes: c, E^, and E®, after Klein and Noble
Smith : D*, after Wiedersheim : F^, after Sachs : f’, after Behrens.)
coloured by a pigment called hcemo^lobin, and provided
eaeh with a large nucleus {tiu) which, when the corpuscle is
seen from the edge (b‘^), jiroduces a bulging of its central part.
These bodies may be compared to Amcebte which have
drawn in their jiseudopods, assumed a flattened form, and
become coloured with haemoglobin.
In the blood of mammals, such as the rabbit, dog, or man,
similar leucocytes occur, but their red blood corpuscles (c-^c^)
have the form of biconcave discs, and are devoid of nuclei.
In many animals the leucocytes have been observed to
ingest solid particles (c^), to multiply by simple fission (b^),
and to coalesce with one another forming plasmodia (b^, b®)
(p- 52)-
The stomach and intestines of animals are lined wuth a
sort of soft slimy skin called mucous membrane. If a
bit of the surface of this membrane — in a frog or rabbit for
instance — is snipped off and ** teased out,” torn apart
with needles, it is found when examined under a high power
to be made up of an immense number of microscopic
bodies called epithelial cells,, which in the living animal, lie
close to one another in the inner layer of mucous mem-
brane in something the same way as the blocks of a wood
pavement lie on the surface of a road. An epithelial cell
(dS d*) consists of a rod-like mass of protoplasm, contain-
ing a large nucleus, and is therefore comparable to an
VI
PARENCHYMA CELLS
S9
elongated Amceba without pseudopods. In some animals
the resemblance is still closer : the ej^ithelial cells have been
observed to throw out pseudopods from their free surfaces
(d^), that is, from the only part where any such movement
is possible, since they are elsewhere in close contact with
their fellow cells.
The mouth of the frog and the trachea or windpipe of air-
breathing vertebrates such as reptiles, birds, and mammals,
are also lined with mucous membrane, but the epithelial
cells which constitute its inner layer differ in one important
respect from those of the stomach and intestine. If ex-
amined quite fresh each is found to bear on its free surface,
/>., the surface which bounds the cavity of the mouth or
windpipe, a number of delicate protoplasmic threads or
cilia (e’ — K^) which are in constant vibratory movement. In
the process of teasing out the mucous membrane some of
the cells, are pretty sure to become detached, and are then
seen to swim about in the containing fluid by the action
of their cilia. These ciliated epithelial cells remind one
strongly of Heteromita, except for the fact that they bear
numerous cilia in constant rhythmical movement instead of
two only — in this case distinguished as flagella — presenting
an irregular lashing movement.
Similar ciliated epithelial cells are found on the gills ot
oysters, mussels, &c., and in many other situations.
The stem or root of an ordinary herbaceous plant, such
as a geranium or sweet- pea, is found when cut across to
consist of a central mass of pith, around whrich is a circle
of woody substance, and around this again a soft greenish
material called the cortex, A thin section shows the latter
to be made up of innumerable polyhedral bodies called
6o
PARENCHYMA CELLS
LESS.
parenchyma cells^ fitting closely to one another like the
bricks in a wall.
A parenchyma cell examined in detail (f^) is seen to
consist of protoplasm hollowed out interfially into one or
more cavities or vacuoles {vac) containing a clear fluid.
These vacuoles differ from those of Amceba, Heteromita, or
Euglena in being non-contractile ; they are in fact mere
cavities in the protoi)lasm containing a watery fluid : the
layer of protoplasm immediately surrounding them is denser
than the rest. Sometimes there is only one such space
occupying the whole interior of the cell, sometimes, as in
the exam])le figured, there are several, se])arated from one
another by delicate bands or sheets of protoplasm. The
cell contains a large nucleus (nu) and is enclosed in a
moderately thick cell-wall composed of cellulose.
The above description applies to the cells composing the
deeper layers of the cortex, />., those nearest the woody
layer i in the more superficial cells, as well as in theinternal
cells of a leaf, there is something else to notice. Imbedded
in the protoplasm, just within the cell-wall, are a number of
minute ovoid bodies of a bright green colour (f^, chr\
These are chromatophores or chlorophyll corpuscles ; they
consist of protoplasm coloured with chlorophyll, which can
be proved experimentally to have the same properties as the
chlorophyll of Hoematococcus and Euglena.
Such a green parenchyma cell is clearly comparable with
an encysted Haematococcus or Euglena, the main differences
being that in the plant-cell the form is polyhedral ownng to
the pressure of neighbouring cells and that the chromato-
phores are relatively small and numerous. Similarly a
colourless parenchyma cell resembles an encysted Amceba.
The pith, the epidermis or thin .skin which forms the
outer surface of herbaceous plants, the greater part of the
VI
MINUTE STRUCTURE OF CELLS
6i
leaves, and other portions of the plant may be shown to
consist of an aggregation of cells agreeing in essential
respects with the above description.
We come therefore to a very remarkable result. 1'he
higher animals and plants are built up— in i)art at least — of
elements which resemble in their essential features the
minute and lowly organisms studied in previous lessons.
Those elements are called by the general name of cells:
hence the higher organisms, whether plants or animals, are
multicellular or are to be considered as cell a^gregateSy
while in the case of such beings as Ama'ba, Hsematococ-
cus, Heteromita, or Euglena, the entire organism is a
single cell, or is unicellular.
Note further that the cells of the higher animals and
plants, like entire unicellular organisms, may occur in either
the amoeboid (Fig. 8, a, c^) the ciliated (k), or the
encysted (f) condition, and that a plasmodial phase (b") is
sometimes produced by the union of two or more amoeboid
cells.
One of the most characteristic features in the unicellular
organisms described in the preceding lessons is the con-
stancy of the occurrence of binary fission as a mode of
multiplication. The analogy between these organisms and
the cells of the higher animals and plants becomes still
closer when w^e find that in the latter also simple fission is
the normal mode of multiplication, the increase in size of
growing parts being brought about by the continual division
of their constituent cells.
The process of division in animal and vegetable cells
is frequently accompanied by certain very characteristic and
complicated changes in the nucleus to which we must now
62
MINUTE STRUCTURE OF CELLS
LESS.
direct our attention. First of all, however, it will be neces-
sary to des('iil)e the exact microscopic structure of cells and
their nuclei as far as it is known at present.
Fk;. 9. — A, Cell fioni llie genital lidge ol a young salamander,
showing rell-memhrane {f. ///), protoplasm or cell -body (r. />) with
astiosj)here (s) and centiosome (r), and nucleus with membrane
(;///. w) and inegular network of chromatin {r/tr). B. Cell from the
immatuie stamen of a lily, show'ing cell-wall (e. w), protoplasm, with
nucleus as in A. (Tlu‘ astrospheres here figunal are incorrect. —
Roth liguies very highly magnifital.
(a, from a diawing !)>' J. E. S. Moore ; B, after Guignard.)
There seems to be a good deal of variation in the jirecise
structure of various animal and plant cells, but the more
recent researches show that in the cell-body or protoplasm
(Fig. 9, c b) two constituents may be distinguished, a clear
semi-fluid substance, traversed by a delicate sponge-work.
Now under the microscope the whole cell is not seen at
once but only an optical section of it, that is all the
parts which are in focus at one time : by altering the
focus we view the object at successive depths, each view
being practically a slice parallel to the lenses of the
instrument. This being the case, protoplasm presents the
microscopic appearance of a clear or slightly granular
VI MINUTE STRUCTURE OF NUCLEI 63
matrix traversed by a delicate network. In the epithe-
lial cells of animals the protoplasm is bounded exter-
nally by a cell-membrane (Fig. 9, a , c, m) of extreme
tenuity, in plants by a cell-wall ( b , c. w) of cellulose : in
amoeboid cells the ectosarc or transparent non-granular
portion of the cell consists of clear protoplasm only, the
granular endosarc alone possessing the sponge-work. In
the majority of full-grown plant cells (Fig. 8, f) and in
some animal cells the protoplasm is more or less exten-
sively vacuolated, but in the young growing jiarts as well
as in the ordinary cells of animals the foregoing descrijition
holds good. It is quite possible that the reticular character
of the protoplasm may be merely the optical expression of
an extensive but minute vacuolation, or may be due to the
presence of innumerable minute granules developed in the
protoplasm as products of metabolism.
The nucleus is usually spherical in form : it is enclosed
in a delicate nuclear membrane (tiu.ni) and contains, as in
Amoeba (p. 7) two constituents, the nuclear sap and the
chromatin which exhibit far more striking differences than
the two constituents of the cell-body. The nuclear sap
is a homogeneous semi-fluid substance which forms the
ground-work of the nucleus: it resembles the clear cell-
protoplasm in its general characters, amongst other things
in being unaffected by dyes. The chromatin (chr) takes the
form of a network or sponge-work of very variable form,
and is distinguished from all other constituents of the cell
by its strong affinity for aniline and other dyes. Frequently
one or more minute globular structures, the nucleoli (b, nu\
occur in the nucleus either connected with the network or
lying freely in its meshes : they also have a strong affinity
for dyes although they often differ considerably from the
chromatin in their micro-chemical reactions.
Fu'r. 10 .— 1 h.iL^iains illu<^trating the process of indirect cell division
or
A, the cell : the nucleus allows a nuclear membrane {fiK. w\
chrouKitm ai Kinged in loop.', united into a network (the latter
shown on ilie iiglit -ide i>nl\ ), and iwt) nuclo(di [nil): near the nucleus
is an astiosplu'ie t'), containing a ceiUro^oine (r) and surrounded by
radiating protoplasmic lilanu nls.
B, The chromatin has resolved itself into distinct loops or chromo-
somes [c'hr) which have divided longitudinally : the nuclear membrane
has begun to disapjiear : there are two astrospheres and between them
is seen the connnenceinent of the nuclear spindle ('i/).
c, ddie nuclear membrane has disappeared : the cliromosomes are
CKLL-DIVISKJN
LESS. VI
65
aiiangcd irrcj^ularly : llu' spimlle has increased in si/c and is siimlcd
definitely within tlu* nueleai aie.i.
1), The chioinosonu's aic anani;ed lound tlic equati'i* of the fully
formed luiclcat spindle.
E, 'I’he d.m^hit. r-loops (if the chiumosonies are passinj^ in opposite
diieclions towauK the poU‘s of the spindle, each liaving a spindle-fibie
allailicd to it.
F, I ater stage of tlk‘ same piocess.
G, The chromosomes aie now arianged in two distinct gioiips, one at
each jiolc of llic sjnndle.
H, The daughter-cells aie partly seiiaiated l>y consliiction and llu*
chromosomes of each grouj) aie uniting to foim the nelwiak of the
daughter- nucleus.
I, .Sliows the di\ision of a ]»lant cell by the formation of a Cell-])late
(c. //) : the daughter nuclei aie fully formed.
(Altered fiom J'lemming, Rabl, &C. )
In the body of some cells and possibly of all there is
found a globular body, surrounded by a nuliating ariange-
ment of the ptotoplasm and called the astrosplu're (.s) ; it
lies (.lose to the niK-leiis, and contains a minute gianule
known as the ceniral pariiclc or anirosome. (r). In many
cells two astrosiiheres and two or more centrosoiiies ha\e
been found in each cell (n, s\
The precise changes w'hich take jdace during the fission
of a cell arc, like the structure of the cell itself, subject
to considerable variation. We will consider what may
probably be taken as a typical case (Fig. 10).
First of all, the astrospherc, with its centrosome, divides (h)
and the products of its division gradually sc])aratc from one
another (c), ultimately passing to opi)o.sitc iioles of the nucleus
(])). At the same time the network of chromatin divides
into a number of separate filaments called chromosomes (\\ chr\
the number of which appears to be constant in any given
species of animal or plant, although it may vary in different
species from 2 to 168 or more. Soon after this the nuclear
membrane and the free nucleoli disappear (b, c) and the
66
MINUTF STRIJCTURE OF CKLLS
l.ESS.
nucleus is seen to contain a spindle shaped body (jr/) formed
of excessively delicate fibres v^hich converge at each pole
to the corres[)onding astrosphere. The precise origin of
this nuclear spindle is uncertain : it may arise either
from the nuclear matrix or, more probably, from the
protoplasm of the cell ; it is not affected by colouring
matters.
At the .same time each chromosome s[)lits along its whole
length so as to form two ])aiallel rods or loojis in close
contact with one another (h) : in this way the number of
chromosomes is doubled, each one being now rejiresented
by a cou])le.
The divided chromosomes now ])ass to the equator of the
spindle (d) and assume the form of more or less V-shaped
loojis, which arrange themselves in a radiating manner so as
to present a star-like figure w'hen the cell is viewed in the
direction of the long axis of the spindle. Everything is now
ready for division to which all the foregoing processes are
preiiaratory.
'Fhe two chromosomes of each couple now gradually pass
to opposite poles of the spindle (e, f), two distinct groups
being tlius produced ((;) and each chromosome of each
group being tlie twin of one in the other group. Perhaps
the fibres of the spindle are the active agents in this
lirocess, the chromosomes being dragged in opposite
directions by their contraction : on the other hand it is
possible that the movement is due to the contractility of the
chromosomes themselves.
After reaching the poles of the spindle the chromosomes
of each group upite with one another to form a network (h)
around which a nuclear membrane finally makes its appear*
ance (i). In this w^ay two nuclei are produced within a
single cell, the chromosomes of the daughter-nudei^ as well
VI CELL-DIVISrON 67
as their attendant asiros])hcres, being formed by tlic binary
fission of those of the mothcr-tualcus.
But [^ari passu with the process of nuclear division,
fission of the cell-body is also going on. 'I'liis may take
place by a simple process of constriction (n) -in much the
same way as a lump of clay or dough would divide if a looj)
of string were tied round its middle and then tightened -or
by the formation of what is known as a ccU-plate. This
arises as a row of granules formed from the eipiatorial part
of the nuclear spindle (i): the granules extend until the y
form a comjdete Cipiatorial plate dividing the cell-body into
tsvo halves : fission then takes place by the cell-plate sjilit-
ting into two along a plane parallel with its fiat surfaces.^
In plants the cell-})late gives rise to a partition wall of
cellulose which divides the two daughter-cells from one
another.
In some cases the dividing nucleus, instead of going
through the complicated processes just described, divides
by simjfie constriction. We have therefore to distinguish
between direct and indirect nuclear division. To the latter
very elaborate method the name mitosis or karyokinesis i.s
applied : direct division is then distinguished as amitotic.
In this connection the reader will not fail to note the
extreme complexity of structure revealed in cells and their
nuclei by the highest powers of the microscope. When the
constituent cells of the higher animals and plants w^re
discovered, during the early years of the jjrcsent century, by
Schleiden and Schwann, they were looked upon as the ultima
Thule of microscopic analysis. Now the demonstration of
^ It must not be forgotten that the cells, which are necessarily rcpic-
sented in such diagrams as Fig. lo as planes, are really solid bodies,
anti that consetjucnlly the cell-plate represented in the figures as a line
is actually a plane at right angles to the plane of the pai)er.
68
COMPLEXITY OF CELL STRUCTURE
LESS.
the cells themselves is an easy matter, the problem is to
make out their ultimate constitution. What would be the
result if we could get microscopes as superior to those of
to-day as those of to day arc to the primitive instruments of
eighty or ninety years ago, it is impossible even to conjecture.
But of one thing we may feel confident — of the enormous
strides which our knowledge of the constitution of living
things is destined to make during the next half century.
'Phe striking general resemblance between the cells of the
higher animals and })lants and entire unicellular organisms
has been commented on as a very remaVkable fact : there is
another ecpially significant circumstance to which we must
now advert.
All the higher animals begin life as an egg, which is either
passed out of the body of the parent as such, as in most
fishes, frogs, birds, &c., or undergoes the first stages of its
development within the body of the parent, as in sharks,
some reptiles, and nearly all mammals.
The structure of the egg is, in essential respects, the same
in all animals from the highest to the lowest. In a jelly-fish,
for instance, it consists (Fig, 1 1, a) of a globular mass of
proto})lasm in which are de])osited granules of a pro-
teinaceous substance known as yolk-spheruks. Within the
protoplasm is a large clear nucleus {g.v) the chromatin of
which is aggregated into a central mass or nucleolus
An investing membrane may or may not be present. In
other words the egg is a cell : it is convenient, for reasons
which will appear immediately, to speak of it as the ovum
or egg-celL
The young or immature ova of all animals present this
structure, but in many cases certain modifications are under-
gone before the egg is mature, />., capable of development
VI
STRUCT UKK OF TUF FOG
69
into a new individual. For instance, the protoplasm may
throw out pseudopods, the egg becoming amceboid (see
Fig. 52) ; or the surface of the protoplasm may secrete a thick
cell-wall (see Fig. 6 1 ). The most extraordinary modification
takes place in some Vertebrata, such as birds. In a hen’s
egg, for instance, the yolk-si)herules increase immensely,
swelling out the microscopic ovum until it becomes what we
know as the “ yolk ” of the egg : around this layers of
albumen or “white” are deposited, and finally the shell
membrane and the shell. Hence we have to distinguish
carefully in eggs of this cliaracter between the entire “ egg ”
in the ordinary acceptation of the term, and the ovum or
But complexities of this sort do not alter the fundamental
Fro. II, — A, ovum of an animal {Cai'marina hnstaia^ one* (»f the
jelly fishes), showing proto])lasni nucleus and nucleolus (,j;’w).
11, ovum of a plant ( Gymnadenia conopsea, one «)f the orchids), sh«>wing
protoplasm (/Aw), nucleus (««), and nucleolus (//?/').
(A, from Balfour after Haeckel : B, after Marshidl Ward.)
fact that all the higher animals begin life as a single cell, or
in other words that multicellular animals, however large and
complex they may be in their adult condition, originate as
unicellular bodies of microscopic size.
The same is the case with all the higher plants. 1 'he
pistil or seed-vessel of an ordinary flower contains one or
more little ovoidal bodies, the so-called “ ovules ” (more ac-
curately megasporangia — .see Les.son XXXIV., and Fig. 127),
which, when the flower withers, develop into the seeds. A
70
THE PLANT OVUM
T,KSS. VI
section of an ovule shows it to ‘contain a large cavity, the
enihryo-sac or megaspore (see Fig. 126, o), at one end of
which is a microscopic cell and Fig. ii n), consisting as
usual of protoi)lasm nucleus { nu ). and nucleolus
{? ih ). 'Phis is the ovum or egg-cell of the jdant : from it
the new plant, which springs from the germinating seed,
arises. 'Phus the higher plants, like the higher animals,
are, in their earliest stages of existence, microscopic and
unicellular.
LESS. VIII BACTERIUM TERMO 83
esi^ecially in the case of animal infusions, quickly becomes
very strong and disagreeal>le.
The scum after attaining a perceptible t hickness breaks up
and falls to the bottom , and after this the fluid slowly clears
again, becoming once more quite tran.sj)arent and losing its
bad smell. If exposed to the light, ])at ches of green api>ear
in it sooner or later , due to the presence of microscojiic
organisms containing chlorophyll. T'he fluid has acipiired,
in fact, the characteristics of an ordinary stagnant pond, and
is quite incapable of further putrefaction. The whole series
of changes may occui)y many months.
Microscopic examination shows that the freshly-prci)ared
fluid is free from organisms, and indeed, if properly filtered,
i
#
/
/
Fig. 13 —Bacteriuvi termo. A, motile stage : «, resting stage, or
zooghea. (From Klein.)
from particles of any .sort. But the case is very different
when a drop of infusion in which turbidity has set in is
placed under a high power. The fluid is then seen to be
crowded with incalculable millions of minute speckk, only
just visible under a power of 300 or 400 diameters, and all
in active movement. These specks are Bacteria^ or as
they are sometimes called, microbes or viicro-organisms ;
they belong to the particular genus and .species called
Bacterium termo.
Seen under the high power of an ordinary .student’s
microscope Bacterium termo has the appearance shown in
Fig. 13, A : it is like a minute finger-biscuit, i.e. has the form
2
84
BACTERIA
LESS.
of a rod constricted in the middle. But it is only by using
the very highest powers of the microscope that its precise
form and structure can be satisfactorily made out. It is then
seen (Fig. 14) to consist of a little double spindle ^ staining
very deeply with aniline dyes. By the employment of very
high powers it has been shown that t he ])roto])lasm of the
cell contains a nucleus and is covered with a membrane of
extreme tenuity formed either of cellulose or of a proteid
material. According to Dallinger, at each end is attached
a flagellum about as long as the cell itself.
Bacterium termo is much smaller that any organism we
have yet considered, so small in fact that, as it is always
easier to deal with whole numbers than with fractions, its
Fiu. 14. "-Bacterium termo ( x 40(X)), showing the terminal flagella.
(Atier Dallinger.)
size is best expressed by taking as a standard the one-
thousandth of a millimetre, called a micromillimetre and
expressed by the symbol fi. The entire length of the
organism under consideration is from i *5 to 2 /n, i.e. about
the T2 iffiF In other words, its entire
length is n ot more than one-fourth the diameter of a veast-
cell or of a human blood-corpuscle. The diameter of the
flagellum has been estimated by Dallinger to be about I /x
or ^ smallness of which it is as difficult to form
any clear conception as of the distances of the fixed stars.
Some slight notion of these almost infinitely small dimen-
sions may, however, be obtained in the following way. Fig.
14 shows a Bacterium termo magnified 4000 diameters, the
vin
BACILLUS
85
scale above the figure representing inm. magnified to the
same amount. The Iieight of this book is a little over 1 8 cm. ;
tills multiplied by 4,000 gives 72,000 cm. = 720 metres - 2362
feet. AVe therefore get the proportion — as 2362 feet, or
nearly six times the height of St. Paiirs, is to the heidit of
the present volume, so the length of Fij.^ 1 4 is I0 that of
Bacterium terino.
It was mentioned above that at a certain stage of piitre-
fiiclion a scum forms on the surface of the fluid. This film
consists of innumerable motionless Bacteria imbedded
in a transparent trelatinous substance formed of a nr oteid
material (Fig. 13, n). After continuing in the active con-
dition for a time the Bacteria rise to the surfice, lo.se their
flagella, and throw out this gelatinous substance in which
they lie imbedded. 1'he bacterial jelly thus formed is called
a zoo^hea . Thus in Bacterium lermo, as in so many of the
organisms we have studied, there is an alternation of an
active with a resting condition.
•During the earlier stages of putrefaction Bacterium termo
is usually the only organi.sm found in the fluid, but l ater on
othef microbes make their appearance. Of these the com-
monest are distinguished by the generic names Micrococcus ^
Bacillu s, Vibr io^ and Spirillum .
Micrococcus (Fig. 15) is a minute form, the cells of which
are about 2fx mm.) in diameter. It differs from
Bacterium in being g lobular instead of snindlc shapcd and
in having no motile phase. Like Bacterium it assumes the
zooglaea condition {Fig. 15, 4).
Bacillus is commonly found in putrescent infusions in
which the process of decay has gone on for some days : as
its numbers increase those of Bacterium termo diminish.,
86
fJACTKKIA
LESS.
until liarillus becomes the dominant form. Its cells (Fig.
rod sha})ed and about 6 /a (j mm.) in length in the
commonest species. B oth motionless and active forms arc
foundj the latter having a flagellum at each end. The
zoogkea condition is often assumed, and the rods are fre-
(ILicntly found united end to end so as to form filaments.
Vi/>rio resembles Bacillus , but the rod- like cells { Fig. 1 7, a)
are wavy instead of straiaht. 'I'hey are actively motile and
when highly magnified are found to be provided with a
4
\
\ / W
Pio. 15. MunHOiCtt.\. I, single and double (dumb-bell shaped)
foi ms ; 2 and 3, chain-forms : 4, a zooghea.
flagellum at each end. Vibriones vary from 8/a to 25/A in
length.
Spirillum is at once distinguished by its spiral form, the
cells re.sembling minute corkscrews (Fig. 17, b & c) and
being provided with a flagellum at each end (c). The
smaller sj^ecies, such as S. tenue (b) are from 2 to 5 /i in
length, but the larger forms, such as S. volutans (c) attain a
length of from 25 to 30/A. In swimming Spirillum apt^ears
on a superficial examination to undulate like a worm or a
serpent, but this is an optical illusion ; the spiral is really a
permanent one* but during progression it rotates upon its
HINAKV MSSlOX
VIII
S7
long axis, like HcXinalococciis (p. 25), and this double move-
ment produces the appearance of undulation.
Afost Bacteria are colourless, but three species (
vindt\ B, c/ilorinum , and Badllu< vin'fis \ contain chlorophyll ,
and several others form ]>igments of varying tints and often
of great intensity. For instance, t here are red, vellow »
brown , blue , and violet specu'es of iNlicrococcus whii h giow
Flo, iCf,— Bacillus subtilise shcjrwing vaiious stages between single
forms and long filaments {Leptothri.x).
on slices of boiled jjotato, hard-boiled egg, ^:c., forming
brilliantly coloured j)atches ; and the yelknv colour often
assumed by milk after it has been allowed to stand for a
considerable time is due to the presence of Backriinn
xanthinum .
All Bacteria multiolv by simple transver.se fission, the
process taking place sometimes during the motile, sometimes
during the resting condition. Frequently the daughter-cells
do not separate completely from one another but rema in
VIII
NATUKK OK OKNKKIC FORMS
89
remaining: in contact , form a lun^^ wavy or twisted filament
called Lepfothrix (Fig. 16) the separate elements of which
are usually only visible after staining.
I lacilhis also multii)lies by a iicculiar process of spore -
formation which may take place either in the ordinary resting
form or in a [.eptothrix filament. A bright dot appears at
one place in the protoplasm (Kig. 18) : tin’s inert'ust^s in si/e,
the greater jiart of the protojilasm being iisetl U|) in its
formation, and finally takes on the form of a clear oval ^
spore which remains tor some time enclosed in the cell wall
of the bacillus, by the rupture of which it is finally liberated.
In other Bacteria spores are formed directly from the ordin-
ary cells which become thick walled. 1’hc spores difi’er
from the Bacilli in being unstained by aniline dyes.
After a period of rest the si)ores, under favourable cir-
cumstances, germinate by growing out at one end so as to
become rod -like, and thus finally assuming the form of
ordinary Bacilli,
There arc other genera often included among Bar:tcria, for
the description of which the student is referred to the more
special treatises,' One remark must, however, be made in
concluding the present brief account of the mori)hoIogy of
the groiij). There is a great deal of evidence t o show that
what have been spoken of as genera (I^acterium, Bacillus,
Spirillum, &c.) may merge into one another and are there -
fore to be looked upon as )>hases in the life-history of
various microbes rather than as true and distinct genera .
But this is a point which cannot at present be considered
as settled.
The conditions of life of Bacteria are very various. Some
live in water, such as that of stagnant jxjnds, and of the.se
^ See especially Dc Bary, Furtgi, Myceiozoa^ and liacteria (Oxford,
1887), and Klein, Micro-organisms and Disease (London, 1896).
90
BACTKKIA
I.KSS
three species, as already stated (p. 87), contain chlorophyll.
The nutrition of such forms must obviously be holophytic ,
and in the case of llactcrium chlorinuin the giving off of
oxygen in sunlight has actually been i)roved.
But this mode of nutrition is rare among the Bacteria :
nearly all of thos e to w hi ch reference ha s b een in ade_ar^
Fig. 1 S. •— Spf )rc-formation in Bat Ulus. ( From Klein. )
saprophytes, that is, live upon decomposing animal and
vegetable matters. They are, in fact, nourished in precisely
the same way as Heteromita (see p. 37). Many of these
forms, such as Bacterium lermo and species of Bacillus,
Vibrio, &c., will, however, flourish in Pasteur's solution, in
which they obtain their nitrogen in the form of ammonlunL
VIII
BACTKKIA AS KKKMKNTS
91
t artrate instead of decoinDOsintr protcid. It lias also been
shown that s ome Bacteria can .i;o further and make use of
nitrates as a source of nitrogen , and of a carbonate or even
of carbon dioxide as a source of carbon : in other words,
they are able to live upon purely inorganic matter in spile
of the fact that they contain no chlorophyll. Some species
may even multiply to a considerable extent in dislilletl water.
But pari passu with their ordinary nutritive processes,
many Bacteria exert an action on the fluids on which
they live comparably to that e.xerted on a saccharine
solution by the yeasPplant. Such microbes are , in fact .
organized ferments (sec p. 81).
Every one is familiar with the turning sour of milk. 'I'his
change is due to t he conversion of the milk-sugar into
l actic acid .
— 2(C3H(.()3),
Sugar. Lactic Acitl.
The transformation is brought about by the ag enc y of
Bacterium lactis ^ a microbe closely resembling B. termo.
Ijcer and wine are two other fluids which frequently turn
sour, there being in this case a conversion of alcohol into
acetic acid, represented by the equation —
+ O3 - HoO + CgH^O^,
Alcohol. Oxygen. Water. Aceiic Acid.
The ferment in this instance is Bacterium aceti, often
called Mycoderma acetic or the “vinegar plant.” It will
be noticed that in this case oxygen enters into the reaction :
it is a case of fermentation by oxidation.
Putrefaction itself is another instance of fermentation
induced by a microbe . Bacterium termo — the Dutrcfactive
ferment — causes the decomposition of protcids into simpler
compounds, amongst which are such gases as ammon ia
92
BACTERIA
KRSS.
( NIJ.J , sulphuretted hydrogen and ammonium
sulphide ( (NH^).>S) , the evolution of which produces the
characteristic odour of putrefaction.
The fitial stage in putrefaction is the formation of nitrates
and nitrites. The process is a double one, both stages
being due to special forms of Bacteria. In the first place ,
by the agency of the nitrous ferment , ammonia is converted
into nitrous acid —
NH3 + 3O - H,0 4- HNO,
Ammonia. Oxygen. Water. Nitrous Acid.
The nitric ferment then comes into action , converting the
nitrous into nitric acid —
NltOj + O = HNO3
Nitrous Acid. Oxygen. Nitric Acid.
This process is one of vast import ance , since by i ts agency
the soil is constantly receiving fresh s uppli es of nitric acid
which is one of the most important substance s used as
food bv ].)lants.
Besides holQ])hytes and saprophytes there arc included
among Bacteria many parasites^ that is, species which feed
not on decomposing but on living organisms. Many of the
most deadly infectious diseases, such as tuberculosis, diph-
theria, typhoid fever, and cholera, are due to the presence
in the tissues or fluids of the body of particular species of
microbes, which feed upon the parts affected and give rise
to the morbid symptoms characteristic of the disease.
Some Bacteria, like the majority of the organisms pre-
viously studied, require free oxygen for their existence, but
others, like Saccharomyces during active fermentation (see
p. 80), are quite independent of free oxygen and must there-
fore be able to take the oxygen, without which their metabolic
VIII
CONDITIONS OF LIFE
93
processes could not go on, from some of the comi)ounds
contained in the fluid in which they live. Ilacteria arc for
this reason divided into aerobic species which reciuire free
oxygen, and anaerobic species which do not .
As to temi)erature, common observation tells us that
Dacteria flourish only within certain limits. We know for
instance that organic substances can be preserved from
putrefaction by being kept either at the freezint>: ]>oint, or at
or near the boilintr-noint . One important branch of modern
industry, the trade in frozen meat, depends upon the fact that
the putrefactive Bacteria, like other organisms, are rendered
inactive by freezing, and every housekeeper knows how easily
putrefaction can be staved off by roasting or boiling. Simi-
larly it is a matter of common observation that a moderately
high temperature is advantageous to these organisms, the
heat of summer or of the tropics being notoriously favourable
to putrefaction. In the case of Bacterium termo, it has been
found that the optimum teninerature is from to C ..
but that the microbe will flourish between s*' and 40'* C »
Although fully-formed Bacteria, like other organisms, are
usually killed by exposure to heat several degrees below
boiling-point, yet the s{)orc.s of some species will withstand,
at any rate for a limited time, a much higher temperature - -
even one as high as 130® C. On the other hand, j>utrcfactive
Bacteria retain their power of dcvelo])ment after being
exposed to a temperature of -i 1 1° C., although during the
time of exposure all vital activity is of course suspended.
Bacteria also resemble other organisms in being unable
to carry on active life without a due supply of water: no
perfectly dry substance ever putrefie s. T’he pre.servation for
ages of the dried bodies of animals in such countries as
Egypt and Peru dejxjnds at least as much uix)n the moisture-
less air as uix)n the antiseptics used in embalming.
94
BACTERIA
LKss. vm
For the most ])art Hacteria are unaffected by light, since
t hey ^rovv equally well in darkness and in ordinary daylight .
Many of them, however, will not bear [>rolongctl exposure to
direct sunlight, and it has been found possible to arrest the
1 >u trefaction of an organic infusion insolaiion, or exposure
t o the direct action of the sun*s rays. It has also been
proved that it is the light rays and not the heat-rays which
are tiuis prejudicial to the life of micro o rganisms.
LESSON IX
RlOdENFSIS AND AlilOr.KNESIS : H0MC)(;KNKS1S AN 1> HK/I'KRO-
<;kni.sis
Tiie: study of the foivgoing living things and csj)ccially of
Bacteria, the smallest and probably the simplest of all known
organisms, naturally leads us to the consideration of one of
the most i ni)Oitant problems of biology -the problem of
the origin of life.
In all the higher organisms we know that each individual
arises in some way or other^froin a pre-existing individual ;
no one doubts that every bird now' living arose by a i)rocess
of development from an egg formed in the body of a
jjarent bird, and that every tree now’ growing took its origin
either from a seed or from a bud produced by a jmrent idant.
But there have always — until quite recently, at any rate-'
been upholders of the view that the lower forms of life,
bacteria, monads, and the like, may under certain circum-
stances originate independently of pre-existing organisms :
that, for instance, in a flask of hay infusion or mutton broth,
boiled so as to kill any living things present in it, fresh
forms of life may arise de noiv^ may in fact be created
then and there.
We have therefore two theories of the origin of the low’cr
96 BIOGENESIS AND IlOMOGENESIS less.
organisms, the theory of Bio,icenesis ^ according to which each
living thing , however simple, arises by a natural process of
buddin^Tf fission, spore-formation, or what not, from a parent
organism : and the theory of Abiot^eyiesiS s or as it is some-
times called Spontaneous or Eijuivocal Generatiofu accord-
ing to which fully formed living organisms sometimes
arise from notdiving matter.
In former times the occurrence of abiogenesis was uni-
versally believed in. The e.\pression that a piece of meat
has ** bred maggots ” ; the ojunion that parasites such as the
gall-insects of ])lants or the tape-worms in the intestines of
animals originate where they are found ; the belief still held
in some rural districts in the occurrence of showers of frogs,
f)r in the transformation of horse-hairs kept in water into
eels ; all indicate a survival of this belief.
Aristotle, one of the greatest men of science of antiquity,
exj)licitly teaches abiogenesis. He states that some animals
“spring from putrid matter,” that certain insects “ spring
from the dew which falls upon ])lants,” that thread-worms
“ originate in the mud of wells and running waters,” that
fleas “ originate in very small portions of corrupted matter,”
and that “ bugs proceed from the moisture which collects
on the bodies of animals, lice from the flesh of other
creatures.”
Little more than 200 years ago one Alexander Ross.
commenting on Sir Thomas Browne’s doubt as to “ whether
mice may be bred by putrefaction,” says, “ so may he doubt
whether in cheese and timber worms are generated ; or if
beetles and wasps in cow’s dung ; or if butterflies, locusts,
grasshoppers, shell-flsh, snails, eels, and such like, be pro-
created of putrefied matter, which is apt to receive the form
of that creature to which it is by formative power disposed.
To question this is to question reason, sense, and experience.
IX PROBLEM LIMITED TO MICROSCOITC FORMS 97
If he doubts of this lot him to Kgypt, and there he will
find the fields swarininuj with ini<'e, begot of the mud of
Nylus, to the great calamity of the inhaliitants.'’
As accurate incpiiries into these matters were made, the
number of cases in which etiuivocal generation was sup*
j)Osed to occur was rapidly diminished. It was a simple
matter when once thought of — to prove, as Redi did in
1638, that no maggots were ever *M)red in meat on which
flies were prevented by wire screens from laying their eggs.
Far more diflicult was the task, also begun in the seventeenth
century, of proving tliat parasites, such as tape-worms, arise
from eggs taken in with the food ; but gradually this pro-
position was firmly established, so that no one of any
scientific culture continued to believe in the abiogenetic
origin of the more highly organized animals any more tlian
in sh^>wers of frogs, or in the origin of geese from
barna('lcs.
But a new phase of the (picstion was opened with the in-
vention of the microscope. In 1683, Anthony van Leeuwen -
hoek discovered Bacteria, and it was soon found that however
carefully meat might be protected by screens, or infusions by
being placed in well-corked or stoppered bottles, putrefaction
always set in sooner or later, and was invariably accom-
panied by the development of myriads of bacteria, monads,
and other low organisms. It was not surprising, considering
the rapidity wdth which these were found to make their
appearance, that many men of science imagined them to be
produced abiogenelically.
Let us consider exactly what this implies, Sup])Ose we
have a vessel of hay-infusion, and in it a single Bacterium.
The microbe will absorb the nutrient fluid and convert it
into fresh protoplasm : it wall divide repeatedly, and, its
progeny repeating the process, the vessel will soon con-
H
98 BIOGENESIS AND IIOMOGENESIS i.css.
tain millions of Bacteria instead of one. This means, of
course, that a certain amount of fresh living protoplasm has
been formed out of the constituents of the hay-infusion,
through the agency, in the first instance, of a single living
Bacterium. I'he question naturally arises. Why may not
the formation of protojfiasm take place independently of
this insignificant speck of living matter ?
It must not be thought that this question is in any way
a vain or absurd one. That living protoplasm has at some
period of the world’s history originated from not-living
matter seems a necessary corollary of the doctrine of
evolution, and is obviously the very essence of the doctrine
of special creation ; and there is no a priori reason why it
should be impossible to imitate the unknown conditions
under which the process took place. At present, however,
we have absolutely no data towards the solution of this
fundamental problem.
But however insoluble may be the question as to how life
first dawned upon our planet, the origin of living things at
the present day is capable of investigation in the ordinary
way of observation and experiment. The problem may be
stated as follows : — Any putrescible infusion — i.e, any fluid
capable of putrefaction — will be found after a longer or
shorter exposure to swarm with bacteria and monads : do
these organisms, or the spores from which they first arise,
reach the infusion from without, or are they generated within
it? And the general lines upon which an investigation
into the problem must be conducted are simple : given a
vessel of any putrescible infusion ; let this be subjected to
some process which, without rendering it incapable of sup-
porting life, shall kill any living things contained in it ; and
let it then be placed under such circumstances that no living
particles, however small, can reach it from without. If,
IX
EXPERIMENTS ON BIOGENESIS
99
after these two conditions have been rigorously complied
with, living organisms appear in the fluid, such organisms
must have originated abiogenetically.
T'o kill any microbes contained in the fluid it is usually
quite sufficient to boil it thoroughly. As we have seen,
protoplasm enters into heat-rigor at a temperature consider-
ably below the boiling-point of water, so that, with an
exception which will be referred to presently, a few minutes’
boiling suffices to sterilize all ordinary infusions, i.e.^ to kill
any organisms they may contain.
Then as to preventing the entrance of organisms or their
spores from without. This may be done in various ways.
One way is to take a flask with the neck drawm out into
a very slender tube, to bpil the fluid in it for a sufficient
time, and then, while ebullition is going on, to closp the
end of the tube by melting the glass in the flame of a
Bunsen-burner or spirit-lamp, thus hermetically sealing the
flask.
By this method not only organisms and their spores are
excluded from the flask but also air. But this is obviously
unnecessary : it is evident that air may be admitted to the
fluid wdth perfect imjnmity if only it can be filtered, that is,
passed through some substance which shall retain all solid
particles however small, and therefore of course bacteria,
monads, and their spores.
A perfectly efficient filter for this purpose is furnished by
cotton-wool. A flask or test-tube is partly filled with the
infusion : the latter is boiled, and during ebullition cotton-
wool is pushed into the mouth of the vessel until a long and
firm plug is formed (Fig. 19). When the source of heat is
removed, and, by the cooling of the fluid, the steam which
filled the upper part of the tube condenses, air passes in to
supply its place, but as it does so it is filtered of even the
n 2
lOO
BIOGENESIS AND IIOMOGENESIS
LESS.
smallest solid particles by having to pass through the close
meshes of the cotton-wool.
Experiments of this sort conducted with proper care have
been known for many years to give negative results in the
great majority of cases : the fluids remain perfectly sterile
for any length of time. But in certain instances, in spite of
the most careful precautions, bacteria were found to appear
Fig. 19. — A Beaker with a number of test-tubes containing putres-
cible infusions and plugged with cotton wool. (From Klein.)
in such fluids ; and for years a fierce controversy raged
between the biogenists and the abiogenists, the latter in
sisting that the experiments in question proved the occurrence
of spontaneous generation, while the biogenists considered
that all such cases were due to defective methods — either to
imperfect sterilization of the fluid or to imperfect exclusion
of germ-containing atmospheric dust.
The matter was finally set at rest, and the biogenists
IX
EXPERIMENTS ON BIOGENESIS
lOI
proved to he in the right, by the important discovery tliat
the spores of bacteria and monads arc not killed by a teuv
peratiirc many degrees higher than is sufficient to destroy the
adult forms : that in fact while the fully developed organisms
are killed by a few minutes* exposure to a temperature of
70° C. the spores are frequently able to survive several
hours’ boiling, and must be heated to 130'’ — 150^ C in
order that their destruction may be assured. It was also,
shown that the more thoroughly the siiores are dried the
more difficult they are to kill, just as well-dried peas are
hardly affected by an amount of boiling sufficient to reduce
fresh ones to a pulp.
This discovery of the high thermal death-ix)int or ultra-
maximum temperature of the spores of these organisms lias
necessitated certain additional precautions in exjierimcnts
with putrescible infusions. In the first place the flask and
the cotton-wool should both be heated in an oven to a
temperature of 150° C., and thus effectually sterilized. I'hc
flask being filled and plugged with cotton-wool is well boiled,
and is then kept for some hours at a temperature of 32° — 38'*
C., the optimum temperature for bacteria. The object of
this is to allow any spores which have not been killed by
boiling to germinate, in other words to pass into the adult
condition in which the temperature of boiling water is fatal.
The infusion is then boiled again, so as to destroy any such
freshly germinated forms it may contain. "J'he same process
is repeated once or twice, the final result being that the
very driest and most indurated spores are induced to ger-
minate, and arc thereupon slain. It must not be forgotten
that repeated boiling does not render the fluid incapable of
supporting life, as may be seen by removing the cotton-wool
plug, when it will in a short time swarm with microbes.
Experiments conducted with these precautions all tell the
102
BIOGENESIS AND HOMOGENESIS
LESS.
same tale : they prove conclusively that in properly sterilized
])Utrcscible infusions , adequately protected from the entrance
of atmospheric germs , no micro organisms ever make their
appearance. So that the last argument for abiogenesis has
been proved to be fallacious, and the doctrine of biogenesis
shown, as conclusively as observation and experiment can
show it, to be of universal application as far as existing
conditions known to us are concerned.
It is also necessary to add that the presence of microbes
in considerable quantities in our atmosphere has been
proved experimentally. By drawing air through tubes
lined with a solid nutrient material Prof. Percy Frankland
vshowed that the air of South Kensington contains about
thirty-five micro-organisms in every ten litres, and by ex-
jiosing circular discs coated with the same substance he was
further able to prove that in the same locality 279 micro-
organisms fall upon one square foot of surface in one
minute.
There is another question intimately connected with that
of Biogenesis, although strictly speaking quite independent
of it. It is a matter of common observation that, in both
animals and plants, like produces like : that a cutting from
a willow will never give rise to an oak, nor a snake emerge
from a hen^s egg. In other words, ordinary observation
teaches the general truth of the doctrine of Homojs^enesis .
But there has always been a residuum of belief in the
opposite doctrine of Hetero,^enesi 5 y according to which the
oiTspring of a given animal or ])lant may be something
utterly different from itself, a plant giving rise to an animal
or vice versd^ a lowly to a highly organised plant or animal
and so on. Perhaps the most extreme case in which hetero-
genesis was once seriously believed to occur is that of
IX flETEROGENESIS 103
the ** barnaclc-geese /* Buds of a particular tree growing
near the sea were said to produce barnacles, and these
falling into the water to develop into geese. This sounds
absurd enough, but, within the last twenty years, two or three
men of science have described, as the result of repeated
observations, the occurrence of quite similar cases among
microscopic organisms. For instance, t he blood-corpuscles
of the silkworm have been said to give ri se to fungi, the
p rotoplasm of the green weed Nitella (see Fig. 44) to
Amoebse and Infusoria (see p. 107), Eiiglcnse to thread -
worms, and so on.
It is proverbially difficult to prove a negative, and it might
not be easy to demonstrate, what all competent naturalists
must be firmly convinced of, that every one of these sup-
po.sed cases of heterogenesis is founded either upon errors
of observation or upon faulty inductions from correct
observations.
Let us take a particular case by way of example. Many
years ago Dr. Dallinger observed among a number of Vorti-
cellae or bell-animalcules (Fig. 25) one which appeared to
have become encysted upon its stalk. After watching it for
some time, there was seen to emerge from the cyst a free-
swimming ciliated Infusor called Amphilcptus^ not unlike a
long-necked Paramcecium (Fig. 20, p. 108). Many ob-
servers would, have put this down as a clear case of hetero-
genesis ; Dallinger simply recorded the observation and
waited. Two years later the occurrence was explained : he
found the same two species in a pond, and watched an
Amphileptus seize and devour a Vorticella, and, after finish-
ing its meal, become encysted upon the stalk of its victim.
It is obvious that the only way in which a case of hetero-
genesis could be proved would be by actually watching the
transformation, and this no heterogenist has ever done ; at
104
BI(;(iKNP:SlS AND I10M0(;KNESIS
LESS.
the most, certain supposed intermediate stages between the
extreme forms have been observed — say, between a Euglena
and a thread- worm — and the rest of the process inferred.
On the other hand, innumerable observations have been
made on these and oilier organisms, the result being that
each species investigated has been found to go through a
definite scries of changes in tlie course of its development,
the ultimate result being invariably an organism resembling
in all essential respects that which formed the starting-point
of the observations : haiglenne always giving rise to Eiiglenai
and notliing else, Jlacteria to llacteria and nothing else, and
so on.
'There are many cases which imperfect knowledgtj might
class unde^ heterogenesis, such as the origin of frogs from
tadpoles or cf jelly-fishes. from polypes (Lesson XXII. Fig.
53), but in these and many other cases the apparently
anomalous transformations have been found to be })art of
the normal and invariable cycle of changes undergone by
the organism in the course of its development ; the frog
always gives rise ultimately to a frog, the jelly-fish to a jelly-
fish. If a frog at one time produced a tadpole, at another a
trout, at another a worm : if jelly-fishes gave rise sometimes
to polypes, sometimes to Infusoria, sometimes to cuttle-
fishes, and all without any regular sequence — that would be
hetcrogenesis.
It is perhaps hardly necessary to caution the reader against
the error that there is any connection between the theory of
heterogenesis and that of organic evolution. It might be
said — if, as naturalists tell us, dogs arc descended from
wolves and jackals and birds from reptiles, w'hy should not,
for instance, thread-worms spring from Euglenje or Infusoria
from Bacteria? To this it is sufficient to answer that the
evolution of one form from another takes place by a series
IX
IIETEROGENESIS
lOS
of slow, orderly, progressive changes going on through a
long series of generations (see Lesson XIIL); whereas
heterogencsis presupposes the casual occurrence of sudden
transformations in any direction — leading to either a less
or a more highly organized form — and in the course of a
single generation.^
^ Apart from .such cov tin twin variations, others, whiclj may l)e
<leseril)e<l as disiontinnoin^ <lo sometimes appear \^ith apparent
suddenness, l)ul not to the extent whieti would l)e re«|uired hy llie
theory of heterogenesis. —
LESSON X
PARAMrECIUM, STYLONYCHIA, AND OXYTRICHA
It will have been noticed with regard to the simi>le uni-
cellular organisms hitherto considered that all are not equally
simple ; that Protamceba (Fig. 2, p. 9) and Micrococcus
(Fig. 15, p. 86) may be considered as the lowest of all,
and that the others are raised above these forms in the scale
of being in virtue of the possession of nucleus or contractile
vacuole, or of flagella, or even, as in the case of Euglena
(Fig. 5, p. 45), of a mouth or gullet.
Thus w'e may speak of any of the organisms already
studied as relatively “ high ” or “ low ” with regard to the
rest : the lowest or least differentiated forms being those
which approach most nearly to the simplest conception of a
living thing — a mere lump of protoplasm : the highest or
most differentiated those in which the greatest complication
of structure has been attained. It must be remembered,
too, that this increase in structural complexity is always
accompanied by some degree of division of physiological
labour, or, in other words, that morphological and physio-
logical differentiation go hand in hand.
We have now to consider certain organisms in which this
differentiation has gone much further ; which have, in fact,
LESS. X
GENERAL CHARACTERS
107
acquired many of the characteristics of the highe'r animals
and plants while remaining unicellular. The study of several
of these more or less highly differentiated though unicellular
forms will occupy the next seven Lessons.
It w^as mentioned above that, in the earlier stages of the
putrefaction of an organic infusion, bacteria only were
found, and that later, monads made their appearance. Still
later organisms much larger than monads are seen, generally
of an ovoidal form, moving about very quickly, and seen by
the use of a high power to be covered with innumerable fine
cilia. These are called ciliate Infusoria^ in contradistinction
to monads, which arc often known as flagellate Infusoria :
many kinds arc common in i)iitrefying infusions, some occur
in the intestines of the higher animals, while others are
among the commonest inhabitants of both fresh and salt
water. Five genera of these Infusoria will form the subjects
of this and the four following Lessons.
A very common ciliate infusor is the beautiful “ slij)per
animalcule,” Faramcecium, which from its comparatively
large size and from the ease with which all essential j)oints
of its organization can be made out is a very convenient and
interesting object of study.
Compared with the majority of the organisms which have
come under our notice it may fairly be considered as gigantic,
being no less than — \ mm. (200 — 26011) in length : in
fact it is just visible to the naked eye as a minute whitish
speck.
Its form (Fig. 20 a ) can be fairly well imitated by making
out of clay or stiff dough an elongated cylinder rounded at
one end and bluntly pointed at the other ; then giving the
broader end a slight twist ; and finally making on the side
LESS. K
MOVEMENTS
109
(wf/i) and gullet i^ul ) ; several food vacuoles (/ and the two
contractile vacuoles {<\ vat ).
B, the same in optical sections showing cuticle (f«), cortex {cort)^ and
medulla {med) ; buccal groove {hue. gr\ mouth, and gullet {}inl) ;
numerous food vacuoles (JC vad\ circulating in the direction indicated
by the arrows, and containing particles of indigo, which are finally
ejected at an anal spot ; nieganucleiis (ww), niicronucleus {pa. ;///), and
trichocysts, some of which {trth) arc shown with their thieads ejected.
The scale to the right of this figure aj^plies to a and n.
c, a specimen killed with osmic acid, showing the ejection of triclio-
cysl-threads, which project considerably lieyond the cilia.
I), diagram of binary fission : the niicronucleus {pa. ait) has already
divided, the mega nucleus {tin) is in the act of tlividing.
(n, after Lankester. )
rendered somewhat concave by the twist a wide shallow
groove beginning at the broad end and gradually narrowing
to about the middle of the body, where it ends in a tolerably
deep depression.
The groove is called the Imccal groove (Fig. 20, a ^ h,
bin:, gr ) : at the narrow end is a small aperture, the mouth
which, like the mouth of Euglena (Fig. 5), leads into
the soft internal jirotoplasm of the body. The surface of
the creature on which the groove is placed is distinguished
as the ventral surface, the opposite surface being up})er or
dorsal ; the broad end is anterior, the narrow end posterior,
the former being directed forw^ards as the animalcule swims.
These descriptive terms being decided upon, it will be seen
from Fig. 20 A, that th^uccaji -groove begins on the left side
of the body, j^^tiually curves ovey to the middle of the
ventral face.
As . -iniinal swims its form is seen to be permanent,
exh ped si no contractions of either an amoeboid or a
eug nature. It is however distinctly flexible, often
being be* A i|3L direction when passing between
obstacles such as^!H^[^%ia^^of weed. This perma-
nence of contour is due to the presence of a tolerably firm
though delicate cuticle {cu) which invests the whole surface.
no PARAMCECIUM, STYLONYCHIA, OXYTRICIIA less.
The protoplasm thus enclosed by the cuticle is distinctly
divisible into two portions — an external somewhat dense layer,
the cortical layer or cortex {cort), and an internal more fluid
materia], the viedullary substance or medulla (med). It will be
remembered that a somewhat similar distinction of the
protoplasm into two layers is exhibited by Amoeba (p. 3), the
ectosarc being distinguished from the endosarc simply by
the absence of granules. In Paramoecium the distinction is
a far more fundamental one : the cortex is radially striated
and is comparatively firm and dense, while the medulla is
granular and semi-fluid, as may be seen from the fact that
food particles (/. vac^ see below, p. 112) move freely in it,
whereas they never pass into the cortex. The medulla has
a reticular structure similar to that of the protoplasm of the
ordinary animal cell (Fig. 9, p. 62), consisting of a delicate
granular network the meshes of which are filled with a trans-
parent material. In the cortex the meshes of the network
are closer, and so form a comparatively dense substance.
'Hie cortex also exhibits a superficial oblique striation, form-
ing what is called the myophan layer.
The mouth (yntli) leads into a short funnel-like tube, the
gullet {gul\ which is lined by cuticle and passes through the
cortex to end in the ^|Jfc|||^Mia^tlms making a free com-
munication between water.
The of approxi-
mately equal size, quit^s^^W^H^BFo the entire
animal, and arranged in longitudinal row^over whole
outer surface. They consist of prolon gations of t tex,
and each passes t hrough
They are con^S|||H||||H^^^^^^^^^^Pfhereby
distinguished fr^||^^^^H|||P|iHI^^Rus^uglena,
&c.,^ which exhiblBI^^Wncssintermittent lashing move-
ments (see p. 25, note, and p. 59). Their rapid motion and
X
CONTRACTILE VACUOLES
XII
minute size make them somewhat difficult to see while the
Paramoecium is alive and active, but after death they are
very obvious, and look quite like a thick covering of fine
silky hairs.
Near the middle, and close to the cortex, is a large oval
nucleus (B, ;///), which is peculiar in taking on a uniform tint
when stained, showing none of the distinction into chroma-
tin and nuclear sap which is so marked a feature in many of
the nuclei we have studied (see especially Fig. i, p. 2, and
Fig. 9, p. 62). It has also a further peculiarity: against one
side of it in P. caudatum is a small oval structure (/«. mi)
which is also deeply .stained by magenta or carmine. This
is the micronucleus : it is to be considered as a second,
smaller nucleus, the larger body being distinguished as the
meganucleus. In the closely allied P. aurelia^ there arc two
micronuclei.
Hicre are two contractile vacuoles (c. vac), one situated at
about a third of the entire length from the anterior end of
the body, the other at about the same distance from the
posterior end : tliey are in relation with the cortex.
The action of the contractile vacuoles is very beautifully
seen in a Paramcecium at rest : it is particularly striking in a
specimen subjected to slight pressure under a cover glass,
but is perfectly visibly, in' 6fa<^ whjch has merely temporarily
supended its active S^immi^ naiovements. It is then seen
that during the diastole, or pnase of expansion of each vacuole,
a number — about six to ten — of delicate radiating, spindle-
shaped spaces filled with fluid appear round it, like the rays
of a star (upper vacuole in a & b) : the vacuole itself contracts
or perfi^^ completely disappearing from view,
and immedi^iit^iraS^ifeds the i^adiating canals flow together
and re-fill it, becoming themselves emptied and therefore
invisible for an instant (lower vacuole in a & u) but rapidly
112 PARAMOiCIUM, STYLONYCHIA, OXYTRICIiA less.
appearing once more. 'Fhere seems to be no doubt that the
water taken in with the food is collected into these canals,
emptied into the vacuole, and finally discharged into the
surrounding medium.
I'he process of feeding can be very conveniently studied
in Paramcecium by placing in the water some finely-divided
carmine or indigo. When the creature comes into the
neighbourhood of the coloured jiarticles, the latter are swept
about in various directions by the action of the cilia : some
of them are however certain to be swept into the neighbour-
hood of the buccal groove and gullet, the cilia of which all
work downwards, i.e. towards the inner end of the gullet.
The grains of carmine are thus carried into the gullet, where
for an instant they lie surrounded by the water of which it is
full : then, instantaneously, probably by the contraction of
the tube itself, the animalcule performs a sort of gulp, and
the grains with an enveloping globule of water or food-vacuole
are forced into the medullary protoplasm. This process is
repeated again and again, so that in any well-nourished
Paramcecium there are to be seen numerous globular spaces
filled with water and containing, particles of food — or in the
present instance of carmine or indigo. At every gulp the
newly formed food-vacuole pushes, as it were, its predecessor
before it : contraction of the medullary protoplasm also takes
place in a definite direction, and thus a circulation of food-
vacuoles is produced, as indicated in Fig. 20, b, by arrows.
After circulating in this way for some time the water of the
food-vacuoles is gradually absorbed, being ultimately excreted
by the contractile vacuoles, so that the contained particles
come to lie in the medulla itself (refer to figure). The circu-
lation still continues, until finally the particles are brought to
a spot situated about half-way bet\veen the mouth and the
posterior end of the body : here if carefully \vatched they
X
TKICIIOCYSTS
”3
are seen to approach the surface and tlien to he suddenly
ejected. The si)ot in question is therefore to be looked
upon as a potential or aperture for the egestion of
faeces or undigested food-matters. It is a potential and not
an actual anus, because it is not a true aperture l)ut only a
soft place in the cortex through which, by the contractions
of the medulla, solid particles are easily forced.
Of course when Paramcecium ingests, as it usually docs,
not carmine but minute living organisms, the latter are
digested as they circulate through the medullary protoplasm,
and only the non-nutritious ])arts cast out at the anal s})ot.
It has been found by experiment that this infiisor can
digest not only proteids but also starch and perhaps fats.
The starch is probably converted into dextrin^ a carbo-
hydrate having the same formula but soluble
and diffusible. Oils or fats seem to be partly converted
into fatty acids and glycerine. 11ie niilrition of Paramcecium
is therefore characteristic ally hcdozoic.
It was mentioned above (p. no) that the cortex is ra-
dially striated in optical section. Careful examination with
a very high power shows that this appearance is due to the
presence in the cortex of minute spindle-shaped bodies (ji
and c, tnJi) closely arranged in a single layer and perpen-
dicular to the surface. These are called irichocysts.
When a Paramcecium is killed, either by tlie addition of
osmic acid or some other poisonous reagent or by simple
pressure of the cover glass, it frccjucntly assumes a remark-
able appearance. Long delicate threads suddenly appear,
projecting from its surface in all directions (c) and looking
very much as if the cilia had suddenly ]jrotriided to many
times their original length. But these filaments have really
nothing to do wath the cilia ; they are contained under or-
dinary circumstances in the trichocysts, probably coiled up j
I
114 PARAMCECIUM, StYLONVCHiA, OXYTRICHA le«S.
and by the contraction of the cortex consequent upon -any
sudden irritation they are projected in the way indicated.
In Fig. 20 R, a few trichocysts (tnJi) are shown in the ex-
})loded condition, i.e, with the threads protruded. Most
likely these bodies are weapons of ofience like the very
similar structures (nematocysts) found in polypes (see Lesson
XXL Figs. 50 and 51).
Paramcecium multiplies by simple fission, the division of
the body being always preceded by the elongation and
subseciuent division of the mega- and micronuclcus (Fig.
20 , d). Division of the meganucleus is direct, that of the
micronucleus indirect, i,e. takes place by mitosis.
Conjugation also occurs, usually after multiplication by
fission has gone on for some time, but the details and the
results of the process are very different from what are found
to obtain in Heteromita (p. 41). Two Paramoecia come
into contact by their ventral faces (F'ig. 21, a) and the mega-
nucleus nii) of each gradually breaks up into minute
fragments (d — o) which are either absorbed into the proto-
plasm or ejected. At the same time the micronucleus
(;///. nti) divides, by mitosis, and the process is repeated,
the result being that each gamete contains four micro-
nuclei (b). Two of these become absorbed and disappear
(c, mi. nu\ mi. ?tu') ; of the remaining two one is now distin-
guished as the active pronucieus, the other as the stationary
pronncleus. Next, the active pronucleus of each gamete
passes into the body of the other (c) and fuses with its
stationary pronucleu? (d) : in this way each gamete con-
tains a single nuclear body, the conjugation-nucleus (e),
formed by the union of two similar pronuclei one of
which is derived from another individual. It is this
fusion of two nuclear bodies, one from each of the con-
Fio. 21. — uiOv Cofjjui^atuni of Parama'cinm,
A, Commencement of conjiij^^alion : the inet^annelci (w'/. nit) of tiu*
t\yo gametes are almost unaltered : the mieromiclei {ini. nn) are in an
early stage of mitosis : gullet.
n, The micronuclei have divided twdee, each gamete now containing
four.
C, Two of the micronuclei {;///. nu\ mi, nu") of each gamete are
degenerating : of the remaining two, one — the active pronucieiis is
passing into the other gamete.
1), The active pronucleus of each gamete has passed into the other
gamete and is conjugating with its stationary pronucleitK. The mega-
nucleus {ing-. nu) has begun to break up.
E, Each gamete contains a single conjugation-nucleus formed by the
union of its own stationary pronucleus with the active pronucleus of
the other gamete. On the right side the conjugation-nucleus is beginning
to divide.
F, Conjugation is over and only one of the separated gapietcs is shown.
It contains the fragments of the mcganucicus (dotted) and four nuclear
bodies {mi. nu) produced by the division and redi vision of the con-
jugation-nucleus.
G, Two of the products of division of the conjugation-nucleus (d/;'’. nu)
are enlarging to form meganuclei, the other two {Afi. nu) are taking or
the characters of micronuclei. Fragments of the original mcganuclems
nu.) still remain.
(After Hertwig.)
I 2
ii6 rARAMCECIUM, STYLONYCHIA, OXVTRICITA less.
j ligating cells, which is the essential part of the whole
process. Soon after this the gametes separate from one
another and begin once more to lead an independent
existence; the conjugation nucleus of each undergoes
a twice repeated process of division, the infusor thus
acijuiring four small nuclei (f). 'Fvvo of these enlarge
and take on tlic character of nieganiiclei (g, Mg. nu\ the
other two remaining unaltered and having the character of
micronuclei (J//. Thus shortly after the com[jletion
of conjugation each individual contains tw'o mega- and
two micronuclei all derived from the conjugation -nucleus.
Ordinary transverse fission now takes place, as described
in the preceding paragraph, each of the two daughter-cells
having one mega- and one micronucleus, and thus the
normal form of the species is re-acquired.
It will be noticed that, in the present instance, conjuga-
tion is not a process of multiplication : it has been
ascertained that during the time two infusors are conju-
gating each might liave produced several thousand offspring
by continuing to undergo fission at the usual rate. 'Fhe
importance of the process lies in the exchange of nuclear
material between the two conjugating individuals : without
such exchange these organisms have been said to undergo
a gradual process of senile decay characterised by diminution
in size and degeneration in structure.
Another ciliated infusor common in stagnant water and
organic infusions is Stylonychia fuytiluSy an animalcule vary-
ing from -^\mm. to -Jmm.
Like Paramoecium it is often to be seen swimming rapidly
in the fluid, but unlike that genus it frequently creeps about,
almost like a wood-louse or a caterpillar, on the surface
of the plants or other solid objects among which it lives.
X
STKUCTURK OK S r\ LON Vri 1 1 A
07
In correspondence with this, instead of being nearl)
cylindrical, it is flattened on one—tlie \ entraK-sido,
and is thus irregularly plano-convex in trans\cise section
(Fig. 22, c).
Fig. 22. — A, Stylouychia mytilitSy ventral aspect, showing lli*' l-ncral
groove p\) and nunilh {/n/h), two nuclei (;///, ;//p, contr rI ili?
vacuole {c. rnrr), and cilia differentiated into hook-hke* (//. c/), kristle*
like (A r/), platc-like {/’. ci), and fan-like {///. c/) organs.
B, one of the plale-like cilia of the same (/> in A), sliowing its
frayed extremity.
c, liansversc section of Qasti'oslyln^ a form allied to Stylonyrliia,
shovsing buccal groove [hue. yr.), small doisa! cilia (//. c/), luxdc-likc
cilium {Ii. ct), and the vanhjus cilia of the luucal gioove, including an
expanded fan-like oigaii (w. ct). A and n alter (daparede and J.ach-
mann : c after Sterki.
It resembles Paramoecium in general structure (compare
Fig. 22, A, with Fig. 20, a); but owing to the absence of
trichocysts the distinction between cortex and medulla is
iiS l»AkAM(KCIUM, STYLONYCIIIA, OXYTRICHA moss.
less obvious : moreover, it has two mega nuclei {nu) and
only one contractile vacuole {c vac)
Jkit it is in the character of its cilia that Stylonychia
is most markedly distinguished from Paramoecium : these
structures, instead of being all alike both in form and size,
are modified in a very extraordinary way.
On the dorsal surface the cilia are represented only by
very minute processes of the cuticle (c, d. ci) set in longi-
tudinal grooves and exhibiting little movement. It seems
probable that these are to be looked upon as vestigial or
riidhnenfary cilia, as the representatives of cilia which
were of the ordinary character in the ancestors of Stylo-
nychia, but which have undergone partial atrophy^ or
diminution beyond the limits of usefulness, in correspond-
ence with the needs of an animalcule which has taken to
creeping on its ventral surface, instead of swimming freely
and so using all its cilia equally.
On the other hand, the cilia on the ventral surface have
undergone a corresponding enlargement or hypertrophy.
Near the anterior and posterior ends and about the middle are
three groups of cilia of comparatively immense size, shaped
either like hooks (//. ci,\ or like flattened rods frayed at
their ends (/. ^/, and n). All these structures neither vibrate
rhythmically like ordinary cilia nor perform lashing move-
ments like flagella, but move at the base only, like single-
jointed legs. The movement is under the aniinars control,
so that it is able to creep about by the aid of these hooks
and plates in much the same way as a caterpillar by means
of its legs.
Notice that we have here a third form of contractility : in
amoeboid movement there is an irregular flowing of the pro-
toplasm (pp. 4 and 9) ; in ciliary movement a flexion of
a protoplasmic filament from side to side (p, 33); w^hile
X DIFFERENTIATION OF CILIA 119
in the present case we have sudden contractions taking place
at irregular intervals. The movements of these locomotor
hooks and plates are therefore very similar to the muscular
contractions to which the movements of the higher animals
are due : it cannot be said that definite muscles arc present
in Stylonychia, but the protoplasm in certain regions of the
unicellular body is so modified as to be able to j)erform a
sudden contraction in a definite direction. 'J'hc nature of
muscular contraction will be further discussed in the next
Lesson (see p. 130).
The remainder of the ventral surface, with the excei)tion
of the buccal groove, is bare, but along each side of the
margin is a row of large vibratilc cilia, of which three at
the posterior end are modified into long, stiff, bristle like
processes (a, b. ci\
. There is also a special differentiation of the cilia of the
buccal groove {buc. On its left side is a single row of
very large and powerful cilia (a and c, m, ci) which are the
chief organs for causing the food-current as well as the
main swimming- organs : each has the form of a triangular
fan-like plate (c, vi. ci). On the right side of the buccal
groove is a row of smaller but still large cilia of the ordinary
form, and in the interior of the gullet a row of extremely
delicate cilia which aid in forcing particles of food down the
gullet into the medulla.
In Stylonychia and allied genera intermediate forms are
found between these peculiar hooks, plates, bristles, and
fans, and ordinary cilia ; from which we may conclude that
these diverse appendages are to be looked upon as highly
modified or differentiated cilia. Probably they have been
evolved in the course of time from ordinary cilia, and on
the principle that the more complicated or specialized
organisms are descended from simpler or more generalized
120 I»ARAM(i:CIUM, STVLONYCJIIA, OXVTRICHA i.ess. x
forms (see Lesson XIII.), we may consider Stylonychia as
the highly-si)ecializcd descendant of some uniformly-ciliated
progenitor.
A third genus of ciliated Infusoria must be referred
to in concluding the i)resent Lesson. We have seen liow
the nucleus of a Paramoecium which has just conjugated
breaks up and apparently disappears (Fig. 21, a— o).
In Oxyiricha^ a genus closely resembling Stylonychia, the
two nuclei have been found to break up into a large number
of minute granules (Fig 23), which can be seen only after
Fig. 23 . — Oxytricha flava^ killed and stained, showing the frag-
mentation of the nuclei. (After Gruber.)
careful staining and by the use of high magnifying powers.
This process is called fragmentation of the nucleus; in
other cases it goes even further, and the nucleus is reduced
to an almost infinite number of chromatin granules only just
visible under the highest powers. From this it seems very
probable that organisms which, like Protamoeba (p. 9) and
Protomyxa (p. 49), appear non-nucleate, are actually pro-
vided with a nucleus in this pulverized condition, and that
a nucleus in some form or other is an essential constituent
of the cell.
LESSON XI
OPALINA
The large intestine of the common frog often contains
numbers of ciliate Infusoria belonging to two or three
genera. One of these parasitic animalcules, called Opalirm
ranarnm^ will now be described. It is easily obtained by
killing a frog, opening the body, making an incision in the
rectum, and spreading out a little of its blackish contents in
a drop of water on a slide.
Opalina has a flattened body witii an oval outline (Fig.
24, A, b), and full-sized specimens may be as much as one
millimetre in length. The protoplasm is divided into cortex
and medulla and is covered with a cuticle, and the cilia are
equal-sized and uniformly arranged in longitudinal rows over
the whole surface (a).
On a first examination no nucleus is apparent, but after
staining a large number of nuclei can be seen-(n, nu), each
being a globular body (c, i), consisting of a nuclear matrix
surrounded by a membrane and containing a coil or net-
work of chromatin. These nuclei multiply within the body
of the infusor, and in so doing pass through the various
changes characteristic of mitosis or indirect nuclear division
122
OPALINA
I.ESS.
(compare Fig. lo, p. 64, with Fig. 24, c) : the chromatin
Fig. 24. — Opalina ranantm.
A, living specimen, surface view, showing longitudinal vows of cilia.
B, the same, stained, showing numerous nuclei (;/?/) in various stages
of division.
0, 1—6, stages in nuclear division,
p, longitudinal fission.
E, transverse fission.
F, the same in a specimen reduced in size by repeated division.
G, final product of successive divisions.
H, encysted form.
1, uninucleate form produced from cyst.
K, the same after multiplication of the nucleus has begun.
(A — c, after Pfitzner ; D — K, from Saville Kent, after Zeller. )
breaks up (c, 2), a spindle is formed with the chromosomes
across its equator (3), the chromosomes pass to the poles of
XI
PARASITISM
123
the spindle (4, 5), and the nucleus becomes constricted (5)
and finally divides into two (6).
The presence of numerous nuclei in Opalina is a fai t
worthy of special notice. The majority of the organisms
we have studied are uninucleate as well as unicellular : the
higher animals and plants we found (Lesson VI.) to consist
of numerous cells each with a nucleus : Opalina, on the
other hand, is multinucleate but its protoplasm is undivided,
so that it presents a condition of things intermediate be-
tween the unicellular and the multicellular types of structure :
it is most suitably described as no 7 i-ceUular. An api)r()ach to
this condition is seen in Stylonychia, which is unicellular
and binucleate ; but apart from Amceba quaria^ tht‘ only
organisms we have yet studied in which numerous nuclei of
the ordinary character occur in an undivided mass of
protoplasm are the Mycetozoa (p. 52), and in them the
multinucleate condition of the plasmodium is largely due to
its being formed by the fusion of separate cells, while in
Opalina it is due, as we shall see, to the repeated binary
fission of an originally single nucleus.
There is no contractile vacuole, and no trace of either
mouth or gullet, so that the ingestion of solid food is impos-
sible. The creature lives, as already stated, in the intestine
of the frog : it is therefore an mternal parasite^ or endo-
parasite^ having the frog as its host. The intestine contains
the partially-digested food of the frog, and it is by the ab-
sorption of this that the Opalina is nourished. Having no
mouth, it feeds solely by imbibition : whether it performs
any kind of digestive process itself is not certainly known,
but the analogy of other mouthless parasites leads us to
expect that it simply absorbs food ready digested by its host,
upon which it is dependent for a constant supply of soluble
and diffusible nutriment, .
Thus Opalina, in virtue of its parasitic mode of life, is
124
OI’ALINA
LESS.
saved the performance of certain work — the work of diges-
tion, that work ])eing done for it by its host. This is the
essence of internal parasitism : an organism exchanges a free
life, burdened with the necessity of flndingfood for itself, for
existence in the interior of another organism, on which, in
one way or another, it levies blackmail.
Note the close analogy between the nutrition of an internal
parasite like Oi)alina and the saprophytic nutrition of a
monad (p. 39). In both the organism absorbs ])roteids
rendered soluble and diffusible, in the one case by the
digestive juices of the host, in the other by the action of
putrefactive bacteria.
The reproduction of Opalina presents certain points of
interest, largely connected with its peculiar mode of life. It
is obvious that if the Opalina3 simi)ly went on rnultii:>lying,
by fission or otherwise, in the frog’s intestine, the poj)ulation
would soon outgrow the means of subsistence : moreover,
when the frog died there would be an end of the parasites.
What is wanted in this as in other internal parasites is some
mode of multiplication which shall serve as a means of dis-
persal^ or in other words, enable the progeny of the parasite
to find their way into the bodies of other hosts, and so start
new colonies instead of rcn>aining to impoverish the mother-
country.
Opalina multiplies by a somewhat peculiar process of
binary fission : an animalcule divides in an oblique direction
(Fig. 24, d), and then each half, instead of growing to the
size of the parent cell, divides again transversely (e). The
process is repeated again and again (f), the plane of division
being alternately oblique and transverse, until finally small
bodies are produced (o), about length, and
containing two to four nuclei.
If the parent-cell had divided simultaneously into a num-
XI
DEVELOPMENT
125
her of these little bodies the process would have been one of
multiple fibsion : as it is, it forms an interesting link between
simple and multiple fission.
Opalina ranarum multiplies in this way in the spring— />.
during the frog's breeding season. Each of the small pro-
ducts of division (g) becomes encysted (h), and in this
passive condition is passed out with the frog^s excrement,
probably falling on to a water-weed or other acpiatic object.
Nothing further lakes place unless the cyst is swallowx'd by
a tadpole, as must frequently happen when these creatures,
produced in immense numbers from the frogs’ cjggs, browse
upon the water-weeds which form their chief food.
Taken into the tadpole’s intestine, the cyst is burst or
dissolved, and its contents emerge as a lanceolate mass of
protoiilasm (i), containing a single nucleus and covered with
cilia. This, as it absorbs the digested food in the intestine
of its host, grows, and at the same time its nucleus divides
repeatedly (k) in the way already described, until by the lime
the animalcule has attained the maximum size it has also
acquired the large number of nuclei characteristic of the
genus.
Here, then, we have another interesting case of develop*
ment (see p. 43) : the organism begins life as a very small
uninucleate mass of proto])lasm, and, as it increases in size,
increases also in complexity by the repeated binary fission
of its nucleus.
LESSON XII
VORTICKLLA AN1> ZOOTHAMNIUM
The next organism we have to consider is a ciliated infusor
even commoner than those described in the two previous
lessons. It is hardly possible to examine the water of a
pond with any care without finding in it, sometimes attached
to weeds, sometimes to the legs of water-fleas, sometimes to
the sticks and stones of the bottom, numbers of exquisitely
beautiful little creatures, each like an inverted bell with a
very long handle, or a wdne-glass with a very long stem.
These are the well-known ‘‘ bell-animalcules ; the com-
monest among them belong to various species of the genus
Vorticella,
The first thing that strikes one about Vorticclla
(Fig. 25, a) is the fact that it is permanently fixed,
like a plant, the proximal or near end of the stalk
being always firmly fixed to some aquatic object, while to
the distal or far end the body proper of the animalcule is
attached.
But in spite of its peculiar form it presents certain very
obvious points of resemblance to Paramoecium, Stylonychia,
and Opalina. The protoplasm is divided into cortex (Fig.
2 Si c, cort) and medulla {pied\ and is invested with a
Fig. 25. — Vorticella,
A, living specimen fully expanded, showing stalk {st) with axial fibre
{ax. peristome {pe)')^ disc {d)^ mouth {vith)^ gullet C^W/), and
contractile vacuole {c. vac).
B, the same, bent on its stalk and with the disc turned away from the
observer.
C, optical section of the same, showing cuticle (r//), cortex (rer/),
medulla {med), nucleus («?<), gullet several food-vacuoles, and
anus {an), as well as the structures shown in A.
a half-retracted and D' a fully-retracted specimen, showing the
coiling of the stalk and overlapping of the disc by the {leristome.
128
VOKTICELLA AND ZOOTHAMNIUM
LESS.
commenccMncnt of binary fission ; completion of the process ;
Ki*, the bariel-sha])e{l product of division swimming freely in the
direction indicated by the arrow.
„ a specimen dividing into a mega/.ooid and several microzooids {m ) ;
F-, division into one mega- and one microzooid.
c,\ two siages in conjugation showing the gradual alisorption of
the miciogamete (///) into the niegagamete.
Il‘, multiple fission of encysted f(»rm, the nucleus dividing into numer-
ous masses ; spore formed by multiple fission ; ir* - iv, development
of the spore ; is undergoing binary fission.
(k— H after Saville Kent.)
delicate cuticle (ni). There is a single contractile vacuole
(a me) the movements of which are very readily made out
owing to the case with which the attached organism is kept
under observation. There is a meganucleus (////) remarkable
for its elongated band like form, and having in its neighbour-
hood a small lounded micronucleiis. Cilia are also present,
but the way in which they are disposed is very peculiar and
charactciistic. 'Fo understand it we must study the form
of tlie body a little more closely.
The conical body is attached by its apex or proximal end
to the stalk : its base or distal end is expanded so as to form
a thickened rim, the peristome {per\ within which is a plate-
like body elevated on one side, called the disc id) and
looking like the partly raised lid of a chalice. Between the
raised side of the disc and the peristome is a depression, the
mouth (w///), leading into a conical gullet (^’7///).
There is reason for thinking that the whole proximal
region of Vorticella answers to the ventral surface of Tara-
mcecium, and its distal surface with the peristome and
disc to the dorsal surface of the free-swimming genus : the
mouth is to the left in both.
A single row of cilia is disposed round the inner border
of the peristome, and continued on the one hand down the
gullet, and on the other round the elevated portion of the
XII
AXIAL KIBRK
150
disc ; the whole row of cilia thus takes a spiral direction.
The rest of the body is completely hare of cilia.
The movements of the cilia produce a very curious
optical illusion : as one watches a fully-exiianded specimen
it is hardly possible to believe that the peristome and disc
are not actually revolving— a state of things whic'h would
imply that they were discontinuous from the rest of the
body. As a matter of fact the apjiearance is due to the
successive contraction of all the cilia in the same direction,
and is analogous to that produced by a strong wind on a
field of corn or long grass. I'lie bending down of suc-
cessive blades of grass jiroduces a series of waves travelling
across the field in the direction of the wind. If instead of
a field we had a largo circle of grass, and if this were acted
upon by a cyclone, the wave would travel round the circle,
which would then appear to revolve.
Naturally the movement of the circlet of cilia produces a
small whirlpool in the neighbourhood of the Vorticella, as
can be seen by introducing finely-powdered carmine into
the water. It is through the agency of this whirlpool that
food particles arc swept into the mouth, surrounded, as in
Paramoecium, by a globule of water : the food-vacuoles
(/. vac) thus constituted circulate in the medullary proto-
plasm, and the non-nutritive jiarts are finally egested at an
anal spot {an) situated near the base of the gullet.
The stalk {st) consists of a very delicate, transparent,
outer substance, which is continuous with the cuticle of the
body and contains a delicate axial fibre {ax, /.) running
along it from end to end in a somewhat sjjiral direction.
This fibre is a prolongation of the cortex of the body
(c, ax. f.) : under a very high power it appears granular or
delicately striated, the striae being continued into the cortex
of the proximal part of the body.
K
ijo VOKTICKLLA ANH ZOOTiiAMNIUM i r:ss.
A striking charaf tcristic of Vorticella is its extreme
irritability, />., the readiness with which it responds to any
external stimulus (see p. lo). The slightest jar of the
micToscope, the* contact of some other organism, or even a
('urrent of water prodiaed by some free -swimming form like
Paramceciiim, is felt directly by the bell-animalcule, and is
followed by an instantaneous change in the relative position of
its parts. The stalk becomes coiled into a close spiral (i)\ T)“)
so as to have a mere fraction of its original length, and the
body from being bcll-shaj)ed becomes globular, the disc being
withdrawn and the peristome closed over it (i>\ d‘^).
The coiling of the stalk leads us to the consideration ot
the particular form of contractility called muscnlar^ which
we have already met with in Stylonychia (p. 1 1 S). It was
mentioned above that while the stalk in its fully expanded
condition is straight, the axial fibre is not straight, but forms
a very open spiral, /.d*., it does not lie in the centre of
the stalk but at any transverse section is nearer the surface
at one spot than elsewhere, and this point as we ascend the
stalk is directed successively to all points of the compass.
Now suppose that the axial fibre undergoes a sudden con-
traction, that is to say, a decrease in length accompanied by
an increase in diameter, since as we have already seen
(p. 10 ) there is no decrease in volume in protoplasmic
contraction. Inhere will naturally follow a corresponding
shortening of the elastic cuticular substance which forms the
outer layer of the stalk. If the axial fibre were entirely
towards one side of the stalk, the result of the contraction
would be a flexure of the stalk towards that side, but, as its
direction is spiral, the stalk is bent successively in every
direction, that is, is thrown into a close spiral coil.
The axial fibre is therefore a portion of the protoplasm
which possesses the property of contractility in a special
FISSION
XU
degree ; in which moreover contraction takes place in a
definite direction — the direction of the length of the fibre —
so that its inevitable result is to shorten the fibre and con-
setpiently to bring its two ends nearer together. This is the
essential characteristic of a muscular contraction, and the
axial fibre in the stalk of \"orticella is therefore to be looked
upon as the first instance of a clearly differentiated muscle
which has come under our notice.
There are some interesting features in the reproduction of
Vorticella. It multiplies by binary fission, dividing through
the long axis of the body (Fig. 25, k-). Hence it is
generally said that fission is longitudinal, not transverse, as
in Paramieciuni. But on the theory (p. 128) that the peris-
tome and disc are dorsal and the attached end ventral,
fission is really transverse in this case also.
It will be seen from the figures that the process takes place
by a cleft appearing at the distal end (k^), and gradually
deepening until there are produced two complete and full-
sized individuals upon a single stalk I'his state of
things does not last Jong : one of the two daughter-cells takes
on a nearly cylindrical form, keeps its disc and peristome
retracted, and acquires a new circlet of cilia near its j)roximal
end (e^) : it then detaches itself from the stalk, which it
leaves in the sole possession of its sister-cell, and swims about
freely, for a time in the direction indicated by the arrow.
Sooner or later it settles down, becomes attached by its
proximal end, loses its basal circlet of cilia, and develops a
stalk, which ultimately attains the normal length.
The object of this arrangement is obvious. If when a
Vorticella divided, the plane of fission extended down the
stalk until two ordinary fixed forms were produced side by
side, the constant repetition of the process would so increase
K 2
132
VORTICKLLA AND ZOOTFIAMNIUM
LES<^.
the numbers of the s])ecies in a given s[)ot that the food-
supply would inevitably run short. This is prevented by
one of the two sister-cells produced by fission leading a free
existence long enough to enable it to emigrate and settle in
a new locality, where the competition with its fellows will be
less keen. 'J'he ])roduction of these free-swimming zooids
is therefore a means of dispersal (see p. i 24) : contrivances
having this obje<‘t in view' are a very general characteristic
of fixed as of parasitic organisms.
(Conjugation occasionally takes place, and presents certain
peculiarities. A Vorticella divides either into tw'o unequal
halves (t“) or into two ecpial halves, one of W’hich divides
again into from two to eight daughter-cells (I'l). There are
thus jiroduced from one to eight fnicrozooids which resemble
the barrel-shaped form (k*) in all but size, and like it become
detached and sw'im freely by means of a basal circlet of cilia.
After swimming about for a time, one of these microzooids
comes in <'.onta<'t with an ordinary form or me^^azooidy when
it attaches itself to it near the proximal end (cd), and under
goes gradual absorj)tion (o-), the mega- and microzooids
becoming conqiletely and permanently fused. As in Para-
imecium, conjugation is followed by increased activity in
feeding and dividing (p. 116).
Notice that in this case the conjugating bodies or gametes
are not of equal size and similar characters, but one, wdiich
is conveniently distinguished as the muro^atuete ( ~ micro-
zooid) is relatively small and active, while the other or
///ei^axamde ( == megazooid, or ordinary individual) is rela-
tively large and passive. As we shall see in a later lesson,
this differentiation of the gametes is precisely what we get in
almost all organisms with two sexes : the microgamete being
the male, the niegagamete the female conjugating body (see
p- 173)-
XII
MKTAMOKniOSlS
*33
The result of conjugation is strikingly different in the three
cases already studied : in Heteroinita (p. 41) the two gametes
unite to form a zyS^l^> ^ motionless body provided with a
cell-wall, the protoiilasm of which divides into spores : in
raramcecium (p. 1 14) no zygote is formed, conjugation being
a mere temporary union : in Vorticella the zygote is an
actively moving and feeding body, indistinguishable from an
ordinary individual of the sj)ecies.
Vorticella sometimes encysts itself (Fig. 25, td), and the
nucleus of the encysted cell has been observed to break up
into a number of sejarate masses, eac'h doubtless surrounded
by a layer of j)rotoplasm. After a time the cyst bursts, and
a number of small bodies or spores (h-) emerge from it, each
containing one of the prodiu:ts of division of the nucleus.
'Jdiese acetuire a circle of cilia (h^), by means of winch they
swim freely, and they are sometimes found to multiply by
simi>le fission (h^). Finally, they settle down (it^) by the
end at tvhich the cilia arc situated, the attached end begins
to elongate into a stalk this increases in length, the
basal circlet of cilia is lost, and a ciliated peristome and
disc are formed at the free end (h"). In this way the
ordinary form is assumed by a process of development
recalling that which we found to occur in Pleteromita (]). 43),
but w ith an important difference : the free-sw'imming young of
Vorticella (h^), to which the .spores formed by division of
the encysted protoplasm give rise, differ strikingly in form
and habits from the adult. This is exi^ressed by saying
that development is in this case accompanied by a meta^
morphosis, this word, literally meaning simply a change, being
always used in biology to express a striking and fundamental
difference in -form and habit betw^een the young and the
adult ; as, for instance, between the tad])ole and the frog,
or betw'een the caterpillar and the butterfly. It is obviou.s
134
VORTICEI.LA AND ZDOTIIAMNIUM
l.ESS.
that in the present instance metamorphosis is another means
of ensuring dispersal.
In Vorticella, as we have seen, fission results not in the
n.z. A
Fig. 26. — Zoothamnium arbusciila,
A, entire colony, magnified, showing nutritive (//. z) and reproductive
{r. z) /ooids ; a.x. f axial fibre of the stem.
B, the same, natural size.
e, the same, magnified, in the condition of retraction.
l>. nutritive zooid, showing nucleus (////), contractile vacuole [c. vac)^
gullet, and axial fibre (j.r. /).
K, reproductive zooid, showing nucleus [mt) and contractile vacuole
(g vai \ and absence of mouth and gullet.
f’-, two stages in the development of the reproductive zooid.
(After Saville Kent.)
production of equal and similar daughter-cells, but of one
stalked and one free-swimming form. It is however quite
possible to conceive of a Vorticella-like organism in which
the parent cell divides into two equal and similar products,
each retaining its connection with the stalk. If this process
were repeated again and again, and if, further, the plane of
XII
DIMORinilSM
*35
fission were extended downwards so as to include the distal
end of the stalk, the result would be a branched, tree-like
stem with a Vorticella-like body at the end of every branch.
As a matter of fact, this process takes ])lace not in Vorti-
cella itself, but in a nearly allied infusor, the beautiful
Zoothamiiiian, a common genus found mostly in sea-water
attached to weeds and other objects.
Zoothamnium arbuscula (Fig. 26, a) consists of a main
stem attached by its proximal end and giving off at its distal
end several branches, on each of which numerous shortly-
stalked bell-animalcules are borne, like foxgloves or (!antcr-
bury-bclls on their stem. The entire tree is about i cm.
high, and so can be easily seen by the naked eye : it is shown
of the natural size in Fig. 26, n.
We see, then, that Zoothamnium differs from all our
previous types in being a compound organism. The entire
“ tree ” is called a colony or stock., and each separate
bell-animalcule borne thereon is an individual or zooid^
morphologically equivalent to a single Vorticella or
Paramoecium.
As in Vorticella, the stem consists, of a cuticular sheath
with an axial muscle-fibre (ax.f\ which, at the distal end
of the main stem, branche.s, like the stem itself, a prolonga-
tion of it being traceable to each zooid (d). So that the
muscular system is common to the whole colony, and any
shock causes a general contraction, the tree-like structure
assuming an almost globular form (c).
It will be noticed from the figure that all the zooids of
the colony are not alike : the majority are bell-shat>ed and
resemble Vorticellae (a, n. Zy and i>), but here and there are
found larger bodies (a, r. z, and e) of a globular form, with-
out mouth, peristome, or di.se, and with a basal circlet of
cilia. The characteristic band-like nucleus {nu) and the
136 VOKTICKLLA AND ZOOTHAMNIUM ikss. xii
contractile vacuole {c, vac) are found in both the bcll'Shaped
and the globular zooids.
It is to these glol)ular, mouthlcss zooids that the functions
of reproducing the whole colony and of ensuring dispersal
are assigned. 1'hey become detached, swim about freely
for a time, then settle down, develop a stalk and mouth
(f\ f'), and finally, by repeated fission, give rise to the
adult, tree-like colony.
The Zoothamnium colony is thus dimorphic^ bearing indi-
viduals of two kinds : nutritive zooids^ w’hich feed and add
to the colony by fission hut are unable to give rise to a new
colony, and reproductive zooids^ which do not feed while
attached, but are capable, after a period of free existence, of
developing a mouth and stalk, and finally producing a new
colony. Dimorphism is a differentiation of the individuals
of a colony, just as the formation of axial fibre, gullet, con-
tractile vacuole, and cilia are cases of differentiation of the
protoplasm of a single cell.
LESSON XU I
SPKCIKS AND THKIR ORKHN : VWK PRINCIPLES OK
CI-ASSIKICATION
Moke than once in the course of the foregoing lessons we
have had occasion to use the word s/>edes — for instance, in
Lesson 1 . (p. 8) it was stated that there were different
kinds or species of Ania*bae, distinguished by the characters
of their pseudopods, the structure of their nuclei, &c.
We must now consider a little more in detail what we
mean by a species, and, as in all matters of this sort, tlie
study of concrete cxami)les is the best aid to the formation
of clear conceiJtions, we wall take, by way of illustration,
some of the various s})ecies of Zoothamniuni.
I'he kind descrilicd in the previous lesson is called
Zoothamnium arbuscula. As Fig. 26, a, shows, it consists of a
tolerably stout main stem, fiom the distal end of which
spring a number of slender branches diverging in a brush-
like manner, and bearing on short secondary branchlets the
separate individuals of the colony : these are of two kinds,
hell-shaped nutritive zooids, and globular reproductive
zooids, so that the colony is dimorphic.
Zoothavinhan (or, for the sake of brevity, Z.) allernans
Fig. 27, a) is found also in sea-water, and differs markedly
138 SPECIES AND THEIR DKKHN kess.
from Z. arbuscula in the general form of the colony. The
main stem is continued to the extreme distal end of the
colony and terminates in a zooid ; from it branches are
given off right and left, and on these the remaining zooids
are borne. To use Mr. Saville Kent s comparison, Z.arbus-
FlO. 27. — Species of Zoothamniiim. A, Z. altirnans. B, Z.
dichotomum. c, Z. siwplex, l>, Z,, affine, E, Z. nutans. (After
Savillc Kent.)
cula may be compared to a standard fruit tree, Z. alternans
to an espalier. In this sjiecies also the colony is dimorphic.
Z. dichotomum (Fig. 27, B)is also dimorphic and presents a
third mode of branclfing. The main stem divides into two,
and each of the secondary branches does the same, so that
a repeatedly forking stem is produced. The branching of
this species is said to be dichotomous^ while that of Z. alter-
nans is monopodiai^ and that of Z, arbuscula umbellate.
Another mode of aggregation of the zooids is found in Z.
simplex {^\g. 27, c), in which the stem is unbranched and
CiKNUS AND SPKCIKS
XI f I
bears at its distal end about six zooids in a cluster. The
zooids are more elongated than in any of the preceding
species, and there are no special reproductive individuals, so
that the colony is homomorphic.
In Z. affine (Fig. 27, n) the stalk is dichotoinous but is
proportionally thicker than in the preceding species, and
bears about four zooids, all alike. It is found in fresh water
attached to insects and other acjuatic animals.
The last species we shall consider is Z, nutans (Fig. 27, k),
which is the simplest known, never bearing more than two
zooids, and sometimes only one.
A glance at Figs. 26 and 27 will show that these six species
agree with one another in the general form of the zooids, in
the characters of the nucleus, contractile vacuole, &c., in
the arrangement of the cilia, and in the fact that they are all
compound organisms, consisting of two or more zooids
attached to a common stem, the axial fibre of which branches
with it, is continuous throughout the colony.
On account of their possessing these important characters
in common , the species described are placed in the single
^enus Zoothamnium , and the characters summarized in the
preceding paragraph arc called generic characters . On the
other hand the points of difference between the various
s pecies, such as the forking of the stem in Z. dichotomum,
the presence of only two zooids in Z. nutans, and so on, are
called specific characters . Similarly the name ZAWthamnium,
which is common to all the species, is the generic name,
while those which are applied only to a particular species,
such as arbuscula, simplex, &c,, are the specific names. As
was mentioned in the first lesson (p. 8), this method of
naming organisms is known as the Linnean system of
binomial nomenclature.
It will be seen from the foregoing account that by a
140
SPECIKS AND THEIR ORKHN
LESS.
species wc understand an assemhlat^e of
ganisnis, wh ether .sim])lc or compound , wli ich agr ee with one
anot her in all but unessential points, such as the precise
number of zooids in /oothamniuin, which may vary con-
siderably in the same species, an d conve, there fore, witliin
the limits of individual variation. Similarly, what we mean
by a genus is a group of species agree i nt^ wi th one a nother
i n the broad features of their org a nization , but diff erin.^ in
d etail , th e differences being constant .
A comi)arison of the six species described brings out
several interesting relations between them. For instance, it
is clear that Z. arbuscula and Z. alternans are far more
com|)lex, /.c., exhibit greater differentiation of the entire
colony, than Z. simplex, or Z. nutans; so that, within the
limits of the one genus, we have comparatively low or
generalized, and comj)aratively high or si)ecialized species.
Nevertlieless, a little consideration will show that we cannot
arrange the species in a single scries, beginning with the
lowest and ending with the highest, for, although wc should
have no hesitation in placing Z. nutans at the bottom of
such a list, it would be impossible to say whether Z. affine
was higher or lower than Z. simplex, or Z. arbuscula than
Z. alternans.
It is, however, easy to arrange the species into groups
according to some definite system. For instance, if wc take
the mode of branching as a criterion, Z. nutans, affine, and
dichotomum will all be placed together as being dichoto-
mous, and Z. simi)lcx and arbuscula as being umbellate —
the zooids of the one and the branches of the other all
springing together from the top of the main stem : on this
system Z. alternans will stand alone on account of its mono-
podial branching. Or, we may make two groups, one of
dimorphic forms, including Z. arbuscula, alternans, and
xin CREATION AND EVOLUTION I4t
dichotomum, and another of homomorphic species, including
Z. affine, simplex, and nutans. We have tims two very
obvious ways of arranging or llie species of
Zoothamnium, and the (piestion arises —which of these, if
either, is the right one ? Is there any standard by which
we can judge of the accuracy of a given classification of
these or any other organisms, or does the whole thing dej^end
upon the fancy of the classifier, like the arrangement of
books in a library? In other words, are all possible classi-
fications of living things more or less artificial, or is there
su<'h a thing as a natural chnsification ?
Suppose we were to try and classify all the members of a
given family— [)arents and grandparents, uncles and aunts,
cousins, second cousins, and so on. Obviously there are a
hundred ways in which it would be possible to arrange
them — into dark and fair, tall and short, curly-haired and
straight-haired and so on. But is is eciually obvious that all
these methods would be purely artificial, and that the only
natural way, />., the only way to show the real connection of
the various members of the family with one another would
be to classify them according to blood-relationshij), in other
words to let our classification take the form of a genea-
logical tree.
It may be said — what has this to do with the point under
discussion, the classification of the species of Zoothamnium ?
Tliere are t wo theories which attempt to account for the
existence of the innumerable species of living things which
inhabit our earth : the t heory of creation and the theory of
evolution .
According to the theory of creation, all the individuals of^
every species existing at the present day — the tens of
thousands of dogs, oak trees, amoebai, and what not — are
derived by a natural process of descent from a single indi-
142
SPECIKS AND THEIR ORKilN
LESS.
vitlual, or from a j)air of individuals — in each case precisely
resembling, in all essential respects, their existing descend-
ants- which came into existence by a ]>rocess outside the
ordinary course of nature and knowm as Creation. On this
hy})othesis the history of the genus Zoothamnium would be
represented by the diagram (Fig. 28) ; each of the species
being derived from a single individual which came into
Ancestrat Individuals
Fig. 28. — Diagram illustrating the origin of the species of
Zoothamnium by creation.
existence, independently of the progenitors of all the other
species, at some distant period of the earth’s history.
Notice that on this theory the various species are no more
actually related to one another than is either of them to
Vorticella, or for the matter of that to Homo. The in-
dividuals of any one species are truly related since they all
share a common descent, but there is no more relationship
between the individuals of any two independently created
sj)ecies than between any two independently manufactured
XIII
KVOIAJTION
chairs or tables. The words affinity, rclatioiishi]), cVc., as
applied to different species are, on the theory of cre ation,
purely metaphorical, and mean nothing more than that a
certain likeness or community of structure exists ; just as
we might say that an easy chair was more nearly related to a
kitchen chair than either of them to a three legged stool
We see therefore that on the hyjiothesis of creation the
varying degrees of likeness and unlikeness between the
species receive no explanation, and that vfc get no absolute
criterion of classification : we may arrange our organisms,
as nearly as our knowledge allows, according to their resem-
blances and differences, but the relative imjiortance of the
characters relied on becomes a purely subjective matter.
According to the rival theory — that of Descent or Organic
Evolution ~ every species existing at the present day is
derived by a natural process of descent from some other
species which lived at a former period of the world^s
history. If we could trace back from generation to gener-
ation the individuals of any existing species we sliould, on
this hypothesis, find their characters gradually change, until
finally a period was reached at which the differences were so
considerable as to necessitate the jilacing of tlie ancestral
forms in a different species from their descendants at the
present day. And in the same w^ay if w'c could trace back
the species of any one genus, >vc should find them gradually
approach one another in structure until they finally con-
verged in a single species, differing from those now existing
but standing to all in a true parental relation.
Let us illustrate this by reference to Zoothamniiim. As a
matter of fact we know nothing of the history of the genus, but
the comprehension of what is meant by the evolution of species
will be greatly facilitated by framing a working hypothesis.
Suppose that at some distant period of the world’s history
144
SPECIKS AND THEIR ORKHN
LESS,
there existed a Vorticella-like organism which we will call
A (Fig. 29), having the general characters of a single,
stalked zooid of Zoothamnium (compare Fig. 26, F^), and
siij)posc that, of the numerous descendants of this form,
represented by the lines diverging from a, there were .some
in which both the zooids formed by the longitudinal division
of the body remained attached to the stalk instead of one of
them swimming off as in Vorticella. The result — it matters
DIMORPHIC
HOMOAIORPJII
Fig. 29. — L)iagram illustrating the origin of the species 01
Zoothamnium by evolution.
not for our present purpose how it may have been caused —
would be a simple colonial organism consisting of two zooids
attached to the end of a single undivided stalk. Let us call
this form b.
Next let us imagine that in some of the descendants of b,
represented as before by the diverging lines, the plane of
division was continued downwards so as to include the
distal end of the stalk : this would result in the production
XIII
DIVERGENCE OF CHARACTER
145
of a form (c) consisting of two zooids borne on a forked
stem and resembling Z. nutans. If, in some of the descend-
ants of c, this process were repeated, each of the two zooids
again dividing into two fixed individuals and the division
as before affecting the stem, we should get a species (n) con-
sisting of four zooids on a dichotomous stem, like Z. affine.
Let the same process continue from generation to genera-
tion, the colony becoming more and more complex ; we
should finally arrive at a species E, consisting of numerous
zooids on a complicated dichotomously branching stem,
and therefore resembling Z. dichotomum.
Let us further suppose that, in some of the descendants
of our hypothetical form c, repeated binary fission took
place without affecting the stem : the result would be a new
form F, consisting of numerous zooids springing in a cluster
from the end of the undivided stem, after the manner of
Z. simplex. From this a more complicated umbellate form
(g), like Z. arbuscula, may be supposed to have originated,
and again starting from b with a different mode of branch-
ing a monopodial form (h) might have arisen.
Finally, let it be assumed that while some of the descend-
ants of the forms c, d, and f became modified into more
and more complex species, others survived to the present
time with comparatively little change, forming the existing
species nutans, affine, and simplex : and that, in the similarly
surviving representatives of e, g, and h, a differentiation of
the individual zooids took place resulting in the evolution of
the dimorphic species dichotomum, arbuscula, and alternans.
It will be seen that, on this hypothesis, the relative like-
ness and unlikeness of the species of Zoothamnium are
explained as the result of their descent with greater or less
modification or divergence of character from the ancestral
form A : and that we get an arrangement or classification
L
146 SPECIES AND THEIR ORIGIN less.
in the form of a genealogical tree, which, on the hypothesis,
is a strictly natural one, since it shows accurately the
relationship of the various species to one another and to
the parent stock. So that, on the theory of evolution, a
natural classification of any given grou]:> of allied organisms
is simply a genealogical tree, or, as it is usually called, a
phylogeny .
It must not be forgotten that the forms a, b, c, d, e, f, g,
and H are purely hypothetical : they: existence has been
assumed in order to illustrate the doctrine of descent by a
concrete example. The only way in which we could be
perfectly sure of an absolutely natural classification of the
species of Zoothamnium would be by obtaining specimens
as far back as the distant period when the genus first came
into existence ; and this is out of the question, since minute
soft-bodied organisms like these have no chance of being
preserved in the fossil state.
It will be seen that the theory of evolution has the
advantage over that of creation of offering a reasonable
explanation of certain facts. First of all the varying degrees
of likeness and unlikeness of the species are explained by
their having branched off from one another at various
periods : for instance, the greater similarity of structure
between Z. affine and Z. dichotomum than between either of
them and any other species is due to these two species
having a common ancestor in d, whereas to connect either
of them, say with Z. arbuscula, we have to go back to b.
Then again the fact that all the species, however complex in
their fully developed state, begin life as a simple zooid which
by repeated branching gradually attains the adult complexity,
is a result of the repetition by each organism, in the course
of its single life, of the series of changes passed through by
its ancestors in the course of ages. In other words ontogeny^
XIII
HEREDITY AND VARIABILITY
147
or the evolution of the individual , is, in its main features, a
recapitulation of p/iy/o,s^eny or the evolution of the race.
One other matter must be referred to in concluding the
present lesson. It is obvious that the evolution of one
species from another presupposes the occurrence of varia-
tions in the ancestral form. As a matter of fact such
individual variation is of universal occurrence : it is a matter
of common observation that no two leaves, shells, or human
beings are precisely alike, and in our type genus Zootham-
nium the number of zooids, their [>recise arrangement, the
details of branching, &c., are all variables. This may be
expressed by saying that heredity ^ according to which the
offspring tends to resemble the parent in essentials, is
modified by variability ^ according to which the offspring
tends to differ from the parent in details. If from any
cause an individual variation is perpetuated there is produced
what is known as a variety of the species, and, according to
the theory of the origin of species by evolution, such a
variety may in course of time become a new species. Thus
a variety is an incipient species, and a species is a (relatively)
permanent variety.
It does not come within the scope of the present work to
discuss either the causes of variability or those which deter-
mine the elevation of a variety to the rank of a species ;
both questions are far too complex to be adequately treated
except at considerable length, and anything of the nature of
a brief abstract could only be misleading. As a preliminary
to the study of Darwin’s Origin of Species, the student is
recommended to read Romanes’s Evidences of Organic
Evolution, in which the doctrine of Descent is expounded
as briefly as is consistent with clearness and accuracy.
L 2
LESSON XIV
FORAMINIFERA, RADIOLARJA, AND DIATOMS
In the four previous lessons we have learnt how a uni-
cellular organism may attain very considerable complexity
by a process of differentiation of its protoplasm. In the
jiresent lesson we shall consider briefly certain forms of life
in which, while the protoplasm "of the unicellular body un-
dergoes comparatively little differentiation, an extraordinary
variety and complexity of form is produced by the develop-
ment of a ske/efof/j either in the shape of a hardened cell-
wall or by the formation of hard parts within the protojilasm
itself.
The name Foraminifera is given to an extensive group of
organisms which arc very common in the sea, some living
near the surface, others at various depths. They vary in
size from a sand- grain to a shilling. They consist of variously
shaped masses of protoplasm, containing nuclei, and pro-
duced into numerous pseudopods, which are extremely long
and delicate, and frequently unite with one another to form
networks, as at X in Fig. 30. The cell-body of these
organisms is therefore very simple, and may be compared
to that of a multinucleate Amoeba with fine radiating
pseudopods.
LESS. XIV
THE SHELL
149
But what gives the Foraminifera their special character is
the fact that around the protoi)lasm is developed a cell-wall,
sometimes membranous, but usually impregnated with cal-
cium carbonate, and so forming a shelL In some cases, as
in the genus Rotalia (Fig. 30), this is perforated by nume-
rous small holes, through whicli the pseudopods are pro-
truded, in others it has only one large aperture (Fig. 31),
Fig. 30. — A living Foraniinifer {Koialia\ showing the fine ladiating
pseudopods passing through apertures in the chambered slid! : at x
several of them have united. (From Gegenbaur.)
through which the protoplasm protrudes, sending off its
pseudopods and sometimes flowing over and covering the
outer surface of the shell. I'hus while in some cases the
shell has just the relations of a cell-wall with one or more
holes in it, in others it becomes an internal structure, being
covered externally as \rell as filled internally by protoplasm.
The mode of growth of Foraminifera is largely determined
by the hard and non-distensible character of the cell-wall,
150 KORAMINIFERA, RADIOLARIA, DIATOMS less.
which when once formed is incapable of'being enlarged. In
the young condition they consist of a simple mass of proto-
plasm covered by a more or less globular shell, having at
least one aperture. But in most cases as the cell-body
grows, it protrudes through the aperture of the shell as a
mass of protoplasm, at first naked, but soon becoming
covered by the secretion around it of a second compartment
or chamber of the shell. The latter now consists of two
Fio, 31. — A, diagram of a Foramiaifer in which new chambers are
added in a straight line : the smallest first-formed chamber is below,
the newest and largest is above and communicates with the exterior.
n, diagram of a Foraminifer in which the chambers are added in a
flat spiral : the oldest and smallest chamber is in the centre, the newest
and largest as before Communicates with the exterior. (After
Carpenter.)
chambers communicating with one another by a small
aperture, and one of them — the last formed — communi-
cating with the exterior. This process may go on almost
indefinitely, the successive chambers always remaining in
communication by small apertures through which continuity
of the protoplasm is maintained, while the last formed
chamber has a terminal aperture placing its protoplasm in
free communication with the outer world.
XIV COMPl.KXi'rv OF SllFLL 151
The new chambers may be added in a straight line (Fig.
31, a) or in a gentle curve, or in a flat spiral (Fig. 31, b),
or like the segments of a Nautilus shell, or more or less
irregularly. In this way shells of great variety and beauty
Fio. 32. — Section of one of the moic complicated I'oraniinifera
{Alveolina)^ showing the numerous chamhers containing j^rotoplasm
(dotted), separated by partitions of the shell (white). '< 60. (From
Gegenbaur after Car})enter. )
of form are ])roduced, often resembling the shells of Mob
lusca, and sometimes attaining a marvellous degree of com-
plexity (Fig. 32). The student should make a point of
examining mounted slides of some of the })rincipal genera
and of consulting the plates in Carpenter’s I?itroductio?i to
the Study of Foravimifera (Ray Society, 1862), or in Brady's
Report on the Foraminifera of the “ Chaf/enger ” Expedition,
in order to get some notion of the great amount of dif-
ferentiation attained by the shells of these extremely simple
organisms.
152 FORAMINIFERA, RADIOLARIA, DIATOMS less.
The Radiol aria form another group of marine animal-
cules, the numerous genera of which are, like the Foram-
inifera, amongst the most beautiful of microscopic objects.
"J'hey also (Fig. 33) consist of a mass of protoplasm giving
off numerous delicate pscudopods (psd) which usually have
a radial direction and sometimes unite to form networks.
In the centre of the i)rotoplasmic cell-body one or more
nuclei {nu) of unusual size and complex structure are
found.
Fio. 33 . — Lithocircus amtularis, one of the Radiolaria, showing
central capsule {cent. cap . ), intra- and extra capsular protoplasm {int.
cap. pr . , ext. cap. pr . ), nucleus {nu\ pseudopods {psd\ silicious skeleton
{sk€l\ and symbiotic cells of Zooxanthella {z), (After Kiitschli. )
In the interior of the protoplasm, surrounding the nucleus,
is a sort of shell, called the central capsule (cent, caps.),
formed of a membranous material, and perforated by pores
which place the inclosed or intra-capsular protoplasm (int
caps, pr.) in communication with the surrounding or extra-
capsular protoplasm {ext. caps. pr.). But besides this simple
membranous shell there is often developed, mainly in the
extra-capsular protoplasm, a skeleton {skel) formed in the
majority of cases of pure silica, and often of surpassing
beauty and complexity. One very exquisite form is shown
XIV
('nMlu-Al r\ oi’ SIII'IJ.
153
in Fig. 34 : it consists of three pc rfc^r.itud ron('enlne sjiheres
connected by radiating spicules . the material of which it is
composed resembles the <'learest glass.
The student should examine mounted slides of tlu' sili( ions
shells of these organisms — sold undc'r the nanu‘ of Polv-
and should consult the j)lates of IlaedaFs Die
Radiolarieii : he cannot fail to be struck with the complexity
Fig, 34.-- Skclf'tcjii of n Kadiolauan {Jr/immima), consisting of
three concentric perforated splicrc^j tlie two outer partly broken avk.iy
to sliow the inner-— connected by radiating spicules, (loom ( ii g« iil)aur,
after Haeckel.)
and variety attained by the skeletons of organisms which are
themselves little more complex than Amrebae.
Before leaving the Radiolaria, we must touch u])on a
matter of considerable interest connected with the physio-
logy of the group. Imbedded usually in the extra-capsular
154 FORAMINIKERA, RADIOLARIA, DIATOMS less.
protoplasm are found certain little rounded bodies of a
yellow colour, often known as “ yellow cells ’’ (Fig. 33, z).
Each consists of protoplasm surrounded by a cell-wall of
cellulose, and coloured by chlorophyll, with which is asso-
ciated a yellow pigment of similar character called diatomin,
p or a long time these bodies were a comt)lctc puzzle to
biologists, but it has now been conclusively ])roved that they
are independent organisms resembling the resting condition
of Hgematococcus, and called Zooxa 7 itJiella 7 iuiricola.
Thus an ordinary Radiolarian, such as Lithocircus (P'ig.
33), consists of two quite distinct things, the Lithocircus in
the strict sense of the word plus large numbers of Zooxan-
thelhe associated with it. The two organisms multiply quite
independently of one another : indeed Zooxanthella has
been observed to multiply by fission after the death of the
associated Radiolarian.
'This living together of two organisms is known as sym-
biosis. It differs essentially from parasitism (see p. 123), in
which one organism preys upon another, the host deriving
no benefit but only harm from the presence of the parasite.
In symbiosis, on the contrary, the two organisms are in a
condition of mutually beneficial partnership. The carbon
dioxide and nitrogenous waste given off by the Radiolarian
serve as a constant food-supply to the Zpoxanthella : at the
same time the latter by decomposing the carbon dioxide
jirovides the Radiolarian with a constant supply of oxygen,
and at the same time with two important food-stuffs— starch
andproteids — which, after solution, diffuse from the protoplasm
of the Zooxanthella into that of the Radiolarian. The
Radiolarian may therefore be said to keep the Zooxanthellce
constantly manured, while the Zooxanthellae in return supply
the Radiolarian with abundance of oxygen and of ready-
digested food. It is as if a Haematococcus ingested by an
XIV MOVEMENTS OF DIATOMS 155
Amccba retained its vitality instead of being digested : it
would under these circumstances make use of the carbon
dioxide and nitrogenous waste formed as products of kata-
bolism by the Amoeba, at the same time giving off oxygen
and forming starch and protcids. The oxygen evolved would
give an additional supply of this necessary gas to the Amceba,
and the starch after conversion into sugar and the proteids
after being rendered diffusible would in jiart diffuse through
the . cell'Wall of the 1 laematococciis into the surrounding
protoplasm of the Amoeba, to which they would be a
valuable food.
Thus, as it has been said, the relation between a Kadio-
larian and its associated yellow-cells are precisely those
which obtain between the animal and vegetable kingdoms
generally.
The Diatomacem or DtatomSy as they are often called for
the sake of brevity, are a group of minute organisms, in-
cluded under a very large number of genera and species, and
so common that there is hardly a pond or stream in which
they do not occur in millions
Diatoms vary almost indefinitely in form : they may be rod-
shaped, triangular, circular, and so on. Their essential
structure is, however, very uniform : the cell-body contains a
nucleus (Fig. 35, a, nu^ and vacuoles (;vac)^ as well as two
large chromatophores (chr) of a brown or yellow colour;
these are found to contain chlorophyll, the characteristic
green tint of which is veiled, as in Zooxanthella, by diatomin.
The cell is motile, executing curious, slow, jerky or gliding
movements caused by the protrusion of delicate threads of
mucilage from between the valves of the cell-wall : the
threads are shot out at intervals in a given direction, and,
by the resistance of the water, the diatom is jerked in the
opposite direction.
156 FOKAMINIFERA, RADIOI.ARIA, DIATOMS
LESS.
The most interesting feature in the organisation of diatoms
is however the structure of the cell-wall : it consists of two
parts or valves (b, c, c, c. w'), each provided with a rim or
Fig. 35.— a, senii-diagranimatic view of a diatom from its flat face,
showing cell-wall (r. 7v) and protoplasm with nucleus («/^), two vacuoles
{vac)^ and two chromatophores {chr).
B, diagram of the shell of a diatom from the side, turned on its
long axis at right angles to A, showing the two valves (r. w, c. 7v') with
their overlapping girdles.
C, the same in transverse section.
n, surface view of the silicious shell of Nai'icu/a truncata,
K, surface view of the silicious shell of Auiacodisctts sollittiaitus.
(D, after Donkin ; E, after Norman.)
girdle^ and so disposed that in the entire cell the girdle of
one valve {c. w) fits over that of the other {e. w) like the
lid of a pill-box. The cell- wall is impregnated with silica,
so that diatoms can be boiled in strong acid or exposed to
XVI
SEXUAL REPRODUCTION
173
cell, the ovum^ or egg-cell (d^ which, by the gelatiniza-
tion and subsequent disapjiearance of a portion of the wall
of the ovary, is in free contact with the surrounding water.
At the same time the protoplasm of the spermary under-
goes multiple fission, becoming converted into numerous
minute green bodies (d^), each with two flagella, called
spermsr These are liberated by the rupture of the spermary
(d"^) at its distal end, and swim freely in the water.
Some of the sperms make their way to an ovary, and, as
it has been expressed, seem to grope about for the aperture,
which they finally pass through and are then seen
moving actively in the space between the aperture and the
colourless distal end of the ovum. One of them, and prob-
ably only one, then attaches itself to the ovum and be-
comes completely united with it, forming the oosperm? a
body which we must carefully distinguish from the ovum,
since, while agreeing with the latter in form and size, it
differs in having incorporated with it the substance of a
sperm.
Almost immediately the oosperm (d"^, osp) surrounds itself
with a cellulose wall, and numerous oil-globules are formed
in its interior. It becomes detached from the ovary, and,
after a period of rest, germinates (e^, e^) and forms a new
Vaucheria plant.
It is obvious that the fusion of the sperm with the ovum
is a process of conjugation in which the conjugating bodies
differ strikingly in form and size, one — the megagamete or
ovum — being large, stationary, and more or less amoeboid :
the other — the microgamete or sperm — small, active, and
1 Frequently called oosphere.
- Often called spermatozooids or antherozooids.
® Often called oospore.
174 VAUCHERIA AND CAULERPA less.
flagellate. In other words, we have a more obvious case of
sexual diflerentiation than was found to occur in Vorticella,
(p. 132): the large inactive egg-cell which furnishes by far
the greater j)ortion of the material of the oosperm is the
female gamete ; the small active sperm-cell, the function of
which is probably (see Lesson XXIII) to furnish additional
nuclear material, is the male gamete*
Similarly the oosperm is evidently a zygote, but a zygote
formed by the union of the highly differentiated gametes,
ovum and sperm, just as a zygospore (p. 166) is one formed
by the union of equal sized gametes.
As we shall see, this form of conjugation — often distin-
guished as ferti/ization —occxxrs in a large proportion of
flowerless plants, such as mosses and ferns (Lessons XXX.
and XXXL), as well as in all animals but the very lowest.
From lowly water-weeds up to ferns and club mosses, and
from sponges and polypes up to man, the process of sexual
reproduction is essentially the same, consisting in the conju-
gation of a microgamete or sperm with a megagamete or
ovum ; a zygote, the oosperm or unicellular embryo, being
produced, which afterwards develops into an independent
plant or animal of the new generation. It is a truly remark-
able circumstance that what we may consider as the highest
form of the sexual process should make its appearance so
low down in the scale of life.
The nutrition of Vaucheria is purely holophytic ; its food
consists of a watery solution of mineral salts and of carbon
dioxide, the latter being split up, by the action of the chro-
matophores, into carbon and oxygen.
Mucor and Vaucheria are examples of non-cellular plants
which attain some complexity by elongation and branching.
XVI
CAULERPA
J75
The maximum differentiation attainable in this way by a
non-cellular plant may be illustrated by a brief description
of a sea-weed belonging to the genus Caulerpa,
Caulerpa (Fig. 39) is commonly found in rock-pools
between tide-marks, and has the form of a creeping stem
from which root-like fibres are given olf downwards and
branched leaf-like organs upwards. These “ leaves may
attain a length of 30 cm. (i ft.) or more. So that, on a
Fig. 39 . — Caulerpa scalpelliformis (§ nat. size), showing the stem-
like, root-like, and leaf-like portions of the non-cellular plant. (After
Harvey.)
superficial examination, Caulerpa appears to be as complex
an organism as a moss (compare Fig. 39 with Fig. 108, a).
But microscopical examination shows that the plant consists
of a single continuous mass of vacuolated protoplasm,
containing numerous nuclei and green chromatophores and
covered by a continuous cell-wall. Large and complicated
in form as it is, the whole plant is therefore nothing more
than a continuous mass of protoplasm exhibiting no cellular
structure.
LESSON XVII
THK DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS
Hitherto the words “animal” and “plant” have been
either avoided altogether or used incidentally without any
attempt at definition. We are now however in a position to
consider in some detail the precise meaning of the two words,
since in the last half-dozen lessons we have been dealing
with several organisms which can be assigned without hesi-
tation to one or other of the two great groups of living things.
No one would dream of calling Faramoecium and Stylonychia
plants, or Mucor and Vaucheria animals, and we may there-
fore use these forms as a starting-point in an attempt to form
a clear conception of what the names plant and animal really
signify, and how far it is possible to place the lowly organisms
described in the earlier lessons in either the vegetable or the
animal kingdom.
Let us consider, first of all, the chief points of resemblance
and of difference between the indubitable animal Paramcecium
on the one hand, and the two indubitable plants Mucor and
Vaucheria on the other.
In the first place, the essential constituents of all three
organisms is protoplasm, in which are contained one or more
nuclei. But in Paramcecium the protoplasm is invested
LESS. XVII DIFFERENCES IN NUTRITION 177
only by a delicate cuticle interru])ted at the mouth and anus,
while in Mucor and Vaucheria the outer layer is formed by
a firm, continuous covering of cellulose.
We thus have as the first morphological difference between
our selected animal and vegetable organisms t he absence of
a cellulose cell-wall in the former and its presence in the
latter. This is a fundamental distinction, and apiilies
equally well to the higher forms. The constituent cells of
plants are in nearly all cases covered with a cellulose coat
(p. 60), while there is no case among the higher animals of
cells being so invested .
Next, let us take a physiological character. In all three
organisms there is constant waste of substance which has to
be made good by the conversion of food material into proto-
plasm : in other words, con.structive and destructive meta-
bolism are continually being carried on. But when we come
to the nature of the food and the mode of its reception, we
meet at once with a very fundamental difference. In I^ara-
moecium the food consists of living organisms taken whole
into the interior of the body, and the digestion of this solid
proteinaceous food is the necessary prelude to constructive
metabolism. In Vaucheria the food consists of a watery
solution of carbon dioxide and mineral salts — />., it is liquid
and inorganic, its nitrogen being in the form of nitrates or
of simple ammonia compounds. Mucor, like Paramoeciiim,
contains no chloroj)hyll, and is therefore unable to use
carbon dioxide as a food : like Vaucheria, it is prevented
by its continuous cellulose investment from ingesting solid
food, and is dependent upon an aqueous solution. It takes
its carbon in the form of sugar or some such compound,
while it can make use of nitrogen either in the simple form
of a nitrate or an ammonia salt, or in the complex form of
proteids or peptones.
N
178 CHARACTERS OK ANIMALS AND PLANTS
I.ESS.
In this case also our selected organisms agree with animals
and plants generally. Animals, with the exception of some
internal parasites, ingest solid food, and they must all have
their nitrogen siip])lied in the form of proteids, being unable
to build uj) their j^rotoplasm from simpler compounds.
Plants take their food in the form of a watery solution ;
those which i)ossess chlorophyll take their carbon in the
form of carbon dioxide and their nitrogen in that of a nitrate
or ammonia salt : those devoid of chlorophyll cannot, ex-
cept in the case of some Bacteria, make use of carbon
dioxide as a food, and are able to obtain nitrogen either
from simi)lc salts or from proteids. Chlorophyll-lesis pla nts
are therefore nourished partly like green plants , partly
animals.
This difference in the character of the food is connec|:ed
with a morphological difference. Animals have, as
rule, an ingestive aperture or mouth, and some kind of
digestive cavity, cither permanent (stomach) or temporary
(food- vacuole). In plants neither of these structures
exists.
Another difference which was referred to at length in an
early lesson (p. 32), is not strictly one between plants and
animals, but between organisms with and organisms without
chlorophyll. It is that in green plants the nutritive processes
result in deoxidation, more oxygen being given out than is
taken in ; while in animals and not-green plants the precise
contrary is the case.
There is also a difference in the method of excretion. In
Paramoecium there is a special structure, the contractile
vacuole, which collects the superfluous water taken in with
the food and expels it, doubtless along with nitrogenous and
other waste matters. In Vaucheria and Mucor there is no
contractile vacuole, and excretion is simply performed by
XVII
DEFINITIONS
179
diffusion from the general surface of the organism into the
surroundinij medium.
This character also is of some general inportance. 'Fhe
large majority of animals possess a special organ of excretion,
plants have nothing of the kind.
Another difference has to do with the general form of the
organism. Paramcecium has a certain definite and constant
shape, and when once formed produces no new parts.
Vaucheria and Mucor are constantly forming new branches,
so that their shape is always changing and their growth can
never be said to be complete.
Finally, we have what is perhaps the most obvious and
striking distinction of all. Paramcecium possesses in a con-
spicuous degree the power of automatic movement ; in both
Mucor and Vaucheria the organism, as a whole, exhibits no
automatism but only the slow movements of growth. The
spores and sperms of Vaucheria are, however, actively
motile.
Thus, taking Paramcecium as a type of animals, and
Mucor and Vaucheria as types of plants, we may frame the
following definitions : —
Animals are organisms of fixed and definite form, in which
the cell-body is not covered with a cellulose wall. They
ingest solid proteinaceous food, their nutritive processes
result in oxidation, they have a definite organ of excretion,
and are capable of automatic movement.
Plants are organisms of constantly varying form in which
the cell-body is surrounded by a cellulose wall ; they cannot
ingest solid food, but are nourished by a watery solution of
nutrient materials. If chlorophyll is present the carbon
dioxide of the air serves as a source of carbon, nitrogen is
obtained from simple salts, and the nutritive processes
N 2
i8o CHARACTERS OF ANIMALS AND PLANTS less.
result in deoxidation ; if chlorophyll is absent carbon is
obtained from sugar or some similar compound, nitrogen
either from simple salts or from proteids, and the process of
nutrition is one of oxidation. There is no special excretory
organ, and, except in the case of certain reproductive bodies,
there is usually no locomotion.
Let us now apply these definitions to the simple forms
described in the first eight lessons, and see how far they
will help us in placing those organisms in one or other of the
two “ kingdoms” into which living things are divided.
Anueba has a cell-wall, probably nitrogenous, in the
resting condition : it ingests solid proteids, its nutrition being
therefore holozoic : it has a contractile vacuole : and it
performs amceboid movements. It may therefore be safely
considered as an animal.
I Lcmatococcus has a cellulose wall : it contains chloro-
phyll and its nutrition is purely holophytic : a contractile
vacuole is present in H. lacustris : and its movements are
ciliary.
Euglcna has a cellulose wall in the encysted state : in
virtue of its chlorophyll it is nourished by the absorption of
carbon dioxide and mineral salts, but it can also ingest solid
food through a special mouth and gullet : it has a contractile
vacuole, and performs both euglenoid and ciliary move-
ments.
In both these organisms we evidently have conflicting
characters : the cellulose wall and holophytic nutrition
would place them both among plants, while from the con-
tractile vacuole and active movements of both genera and
from the holozoic nutrition of Euglena we should group
them with animals. That the difficulty is by no means
XVII
DOURTFUI. FORMS
easily overcome may be seen from the fact that both genera
are claimed at the present day both by zoologists and by
botanists. For instance, Prof. Huxley considered Hcema-
tococcus as a plant, and ex])ressed doubts about Euglena ;
Mr. Saville Kent ranks Hismatococcus as a plant and
Euglena as an animal ; Prof. Sachs and Mr. Fhiselton
Dyer place both genera in the vegetable kingdom ; while
Profs. Ray Lankester and liutschli group them both among
animals.
In Heteromita the only cell-wall* is the delicate cuticle,
which in the zygote is firm enough to hold the si^ores u\) to
the moment of their escape : food is taken exclusively by
absorption, and nutrition is wholly saprophytic : there is a
contractile vacuole, and the movements arc ciliary.
Here again the characters are conflicting : the probable
absence of cellulose, the contractile vacuole, and the cilia
all have an “ animal ” look, but the mode of nutrition is
that of a fungus.
In Protomyxa there is a decided i)reponderance of animal
characteristics — ingestion of living jirey, and both amoeboid
and ciliary movements. There is no chlorophyll, and the
composition of the cell-wall is not known.
In the Mycetozoa, the life-history of which so closely
resembles that of Protomyxa, the cyst in the resting stage
consists of cellulose, and so does the cell-wall of the spore :
nutrition is holozoic, a contractile vacuole is present in the
flagellulae, and both amoeboid and ciliary movements are
performed. Here again we have a puzzling combination (;f
animal and vegetable characters, and as a consequence we
find these organisms included among plants — under the
name of Myxomveetes or “ slime-fungi ” — by Sachs and
Goebel, w^hile De Bary, Butschli, and Ray Lankester place
them in the animal kingdom.
;S2 CHARACTERS OF ANIMALS AND PLANTS t.ess.
In Saccharomyces there is a clear preponderance of
vegetable characters. The cell-wall consists of cellulose,
nutrition takes place by absorption and proteids are not essen-
tial, there is no contractile vacuole, and no motile phase.
Lastly, in the Hacteria the cell-wall is composed of cellu-
lose, nutrition is usually saprophytic, there is no contractile
vacuole, and the movements are ciliary. So that in all the
characters named, save in the presence of cellulose and the
absence of a contractile vacuole, the Bacteria agree with
Hetcromita, yet they are universally — e xcept by Prof. Claus
— Ijlaced amon^r plants, while Heteromita is as constantly
included among animals.
We see then that while it is quite easy to divide the higher
organisms into the two distinct groups of plants and animals,
any such separation is by no means easy in the case of the
lowest forms of life. It was in recognition of this fact that
Haeckel proposed, many years ago, to institute a third
** kingdom/’ called Protista ^ to include all unicellular organ -
isms. Although open to many objections in practice, there
is a great deal to be said for the proposal. From the strictly
scientific point of view it is quite as justifiable to make three
subdivisions of living things as two : the line between animals
and plants is quite as arbitrary as that between protists and
plants or between protists and animals, and no more so : the
chief objection to the change is that it doubles the difficulties
by making two artificial boundaries instead of one.
The important point for the student to recognise is that
these boundaries are artificial, and that there are no scientific
frontiers in Nature. As in the liquefaction of gases there is
a “ critical point ” at which the substance under experiment
is neither gaseous nor liquid : as in a mountainous country
it is impossible to say where mountain ends and valley
LESSON XX
MONOSTROMA, ULVA, AND NITB:LLA
It was pointed out in a previous lesson (p. 193) that the
highest and most complicated fungi, such as the mushrooms,
are found on analysis to be built up of linear aggregates of
cells — to consist of hyphao so interwoven as to form struc-
<hres often of considerable size and of definite and regular
gTm.
mXhis is not the case with the Algae or lower green plants —
'-■group to which Vaucheria, Caulerpa, Spirogyra, the
san )ms, and, in the view of some authors, Haematococcus
Va^ Euglena, belong. These agree with fungi in the fact
' at the lowest among them {e,g. Zooxanthella) are unicellu-
lar, and others {e.g. Spirogyra) simple linear aggregates ; but
the higher forms, such as the majority of sea-weeds, have,
as it were, gone beyond the fungi in point of structure and
^attained a distinctly higher stage of morphological differen-
tiation. This will be made clear by a study of three typical
genera.
Amongst the immense variety of sea- weeds found in rock-
fools between high and low water-marks are several kinds
aving the form of flat irregular expansions or of bladder-
202
M()N(XSTROMA, ULVA, AND NITELLA
LESS.
like masses, of a bright green colour and very transparent.
One of these is the genus Monostroma^ of which M. bullosum
is a fresh- water species.
Examined microscopically the plant (Fig. 43) is found to
consist of a single layer of close-set, green cells, the cell-walls
of which arc in close approximation, so that the cell-bodies
appear as if embedded in a continuous layer of trans})arent
cellulose. Thus Monostroma, like Spirogyra, is only one
• Fkl 43. — Mouostrofua.
A, surface view of M. bullosum, showing the cells embedded in a
common layer of cellulose : in.any of them are in various stages of
division.
B, vertical section of M. laceratum, showing the arrangement of the
cells in a single layer.
(a after Keinke : B after Cooke.)
cell thick (b), but unlike that genus it is not one but many
cells broad. In other words, instead of being a linear it is
a superficial aggregate.
To use a geometrical analogy : — a unicellular organism
like Hjematococcus may be compared to a point ; a linear
aggregate like Penicillium or Spirogyra to a line ; a superficial
aggregate like Monostroma to a plane.
Growth takes place by the binary fission of the cells (a),
but here again there is a marked and important difference
from Spirogyra. In the latter the plane of division is always
XX
SOLID A(;(iRK(;ATES
203
at right angles to the long axis of the filament, so that growth
takes place in one dimension of space only, namely in length.
In Monostroma the plane of division may be inclined in any
direction provided it is perpendicular to the surface of the
plant, so that growth goes on in two dimensions of space,
namely in length and breadth.
Another of the flat, leaf-like, green sea-weeds is the very
common genus Ulva^ sometimes called “sea-lettuce.” It
consists of irregular, more or less lobed expansions with
crinkled edges, and under the microscope closely resembles
Monostroma, with one important difference : it is formed
not of one but of two layers of cells, and is therefore not a
superficial but a solid aggregate. To return to the geometrical
analogy used above it is to be compared not to a plane but
to a solid body.
As in Monostroma growth takes place by the binary
fission of the cells. But these divide not only along variously
inclined planes at right angles to the surface of the plant
but also along a plane parallel to the surface, so that growth
takes place in all three dimensions of space — in length,
breadth, and thickness.
Ulva may be looked upon as the simplest example of a
solid aggregate, being built up of similar cells, and therefore
exhibiting no celUdifferentiation.
We shall now make a detailed study of a solid aggregate
in which the constituent cells differ very considerably from
one another in form and size, the result being a degree of
complexity far beyond anything w'e have hitherto met with.
Nitella (Fig. 44, a) is a not uncommon fresh-water weed,
found in ponds and water-races, and distinguished at once
Fig.
A, the entire plant (nat. size), showing the segmented stem, each seg-
^ This and the following figures are taken from a New Zealand
species closely allied to, if not identical with, the British N. fiexilis.
LESS. XX
EXTERNAL CHARACTERS
205
iiient {se^ consisting of a proximal internode (/;//. and distal notie
{fiU) : the leaves (/) arranged in whorls and ending in leaflets (/') : the
rhizoids {rh) : and two branches (br), each springing from the axil of a
leaf and ending, like the main stem, in a terminal bud {term. bud).
B, distal end of a shoot with gonads attached to the leaves : ovy^ the
ovaries ; spy^ the spermaries.
C, distal end of a rhizoid.
D, distal end of a leaf (/) with two leaflets (/), showing the chroma-
tophores and the white line. The arrows indicate the direction of rota-
tion of the protoplasm.
E, distal end of a leaflet, showing the general structure of a typical
cell of Nitella in optical section : c. w, the cell-wall ; pls»P^ the quies-
cent outer layer of protoplasm containing chromatophores {chr ) ; pisftr,
the inner layer, rotating in the direction i^idicated by the arrows, and
containing nuclei (nu ) ; vac, the large vacuole.
F, terminal bud, partly dissected, showing the nodes {nd), internodcs
{ini. ttd), and leatwhorls (/), numbered from i to 4, starting from the
proximal end ; gr. pt, growing point.
G, distal end of a leaf (/) with two leaflets (/), at the base of which
are attached a spermary {spy) and two ovaries {ovy).
from such low Algae as Vaucheria and Spirogyra by its ex-
ternal resemblancJe to one of the higher plants, since it
presents structures which may be distinguished as stem,
branches, leaves, &c.
A Nitella plant consists of a slender cylindrical stem,
some 15-20 cm. and upwards in length, but not more thar\
about \ mm. in diameter. The proximal end is loosely
rooted to the mud at the bottom of the stream or pond by
delicate root-filaments or rhizoids (a, rh) : the distal end is
free. Springing from it at intervals are circlets or whorls of
delicate, pointed leaves (/).
Owing to the regular arrangement of the leaves the stem
is divisible into successive sections or segments {seg)y each
consisting of a very short distal division or node {nd) from
which the leaves spring, and of an elongated proximal
division or internode (int nd), which bears no leaves.
Throughout the greater part of the stem the whorls of
leaves are disposed at approximately equal distances from
one another, so that the internodes are of equal length, but
206 MONOSTROMA, ULVA, AND NITELLA LESf
towards the distal end the internodes become rapidly shortei
and the whorls consequently closer together, until, at the
actual distal end, a whorl is found the leaves of which, in-
stead of spreading outwards like the rest, are curled upwards
so that their points are in contact. In this way is formed
the terminal bud (terin. bud\ by which the uninjured stem
is always terminated distally.
The angle between- the stem and a leaf, above (distal to)
the attachment of the latter, is called the ax// of the leaf.
There is frequently fv ‘ springing from the axil of one of
the leaves in a whorl a branch or shoot (br) which repeats
the structure of the main stem, />. consists of an axis from
which spring whorls of leaves, the whole ending in a ter-
minal bud. The axis or stem of a shoot is called a second-
ary axiSy the main stem of the plant being the prhnary axis.
It is important to notice that both primary and secondary
axes always end in terminal buds, and thus differ from the
leaves which have pointed extremities.
The rhizoids or root-filaments {rh) arise, like the leaves
and branches, exclusively from nodes.
In the autumn the more distal leaves present a peculiar
appearance, owing to the development on them of i\\Q gonads
or sexual reproductive organs (Fig. 44, b and c) : of these
the spermaries (antheridia) look very like minute oranges,
being globular structures (spy) of a bright orange colour :
the ovaries (oogonia) are flask-shaped bodies (ovy) of a
yellowish brown colour when immature, but turning black
after the fertilization of the ova.
Examined under the microscope each intemode is found
to consist of a single gigantic cell (f, int nd^) often as much
as 3 or 4 cm. long in the older parts of the plant. A node
on the other hand is composed of a transverse plate of small
XX
HISTOLOGY
207
cells separating the two adjacent internodes from one
another. The leaves consist each of an elongated proximal
cell like an internode (d, /; f, Z^), then of a few small cells
having the character of a node, and finally of two or three
leaflets (d, g, /'), each consisting usually of three cells, the
distal one of which is small and pointed.
Thus the Nitella plant is a solid aggregate in which
the cells have a very definite and characteristic arrange-
ment.
The details of structure of a single cell arc readily made
out by examining a leaflet under a high power. The cell is
surrounded by a wall of cellulose (e, c, 7 Cf) of considerable
thickness. Within this is a layer of protoplasm (primordial
utricle, p. 196), enclosing a large central vacuole (vac), and
clearly divisible into two layers, an outer in im-
mediate contact with the cell- wall, and an inner ( plsm^)
bounding the vacuole.
In the outer layer of protoplasm are the chromatophores
or chlorophyll- corpuscles (chr) to which the green colour of
the plant is due. They are ovoidal bodies, about mm.
long, and arranged in obliquely longitudinal rows (d). On
opposite sides of the cylindrical cell are two narrow ob-
lique bands devoid of chromatophores and consequently
colourless (d). The chromatophores contain minute starch
grains.
The inner layer of protoplasm contains no chlorophyll
corpuscles, but only irregular, colourless granules, many of
which are nuclei (e, tiu\ see below, p. 211). If the tem-
perature is not too low this layer is seen to be in active
rotating movement, streaming up one side of the cell and
down the other (e), the boundary between the upward and
downward currents being marked by the colourless bands
just mentioned, along which no movement takes place (d).
This rotation of protoplasm is a form of contractility very
2o8 monostroma, ULVA, AND NITELLA less. XX
common in vegetable cells in which, owing to the confining
cell' wall, no freer movement is possible.
The numerous nuclei (e, are rod-like and often
curved : they can be seen to advantage only after staining
(Fig. 45). Lying as they do in the inner layer of protoplasm,
they arc carried round in the rotating stream.
In the general description of the plant it was mentioned
that the stem ended distally in a terminal bud (Fig. 44, a,
tervi. bud) formed of a whorl of leaves with their apices
curved towards one another. If these leaves (f, l^) are dis-
sected away, the node from which they spring {nd is found
to give rise distally to a very short internodc (int. nd"^),
above which is a node {nd^) giving rise to a whorl of very
small leaves (/‘^), also curved inwards so as to form a bud.
Within these is found another segment consisting of a still
smaller internode {hit nd'^) and node, bearing a whorl of
extremely small leaves (/^), and within these again a segment
so small that its parts {hit nd^^ / *) are visible only under
the microscope. The minute blunt projections {1% which
are the leaves of this whorl, surround a blunt, hemispherical
projection {gr, pt\ the actual distal extremity of the plant — ‘
the growing point or punctum vegetaiionis.
The structure of the growing point and the mode of
growth of the whole plant is readily made out by examining
vertical sections of the terminal bud in numerous specimens
(Fig. 45).
The growing point is formed of a single cell, the apical
cell (a, ap. c), approximately hemispherical in form and about
^ mm. in diameter. Its cell-wall is thick, and its cell-body
formed of dense granular protoplasm containing a large
rounded nucleus {nu) but no vacuole.
In the living plant the apical cell is continually undergoing
binary fission. It divides along a horizontal plane, a
Fig. 45. — NiteUa : Vertical sections of the growing point at four
successive stages. The nodes (;/^/), internodes {iut. tfd\ and leaf-
whorls (/) are all numbered in onler from the proximal to the distal end
of the bud, the numbers corresponding in all the figures. The proximal
segment {hit. h) in these figures corresponds with the third
.segment {int. nd^^ l^) shown in Fig. 44, F.
In A, the apical cell {ap. c) is succeeded by a very rudimentary node
(niP) without leaves : hit. nd^ is in vertical section, showing the proto-
plasm {plsm), vacuole { 7 )ac\ and two nuclei {tiu).
In B, the apical cell has divided transversely, forming a new apical
cell {ap. c) and a sub-apical cell (r. ap. c) : the leaves (/^^) of nd'^ have
appeared.
In c, the sub-apical cell has divided transversely into the proximally-
situated internode {wt. nd*) and the distally-situated node (nd*) of a
new segment ; in the node the nucleus has divided preparatory to cell-
division. The previously formed segments have increased in size : hit.
fu? has developed a vacuole {vac)y and its nucleus has divided (comp.
int. nd^ in a) : int. neP is shown in surface view with three dividing
nuclei {nu\
In D, nd^ has divided vertically, forming a transverse plate of cells,
and is now as far advanced as nd} in A : the nucleus of int. nd^ is in the
act of dividing, while int, ticP-, .shown in surface view, now contains
numerous nuclei, some of them in the act of. dividing.
P
210
MONOSTROMA, ULVA, AND NITELLA
LESS.
plane parallel to its base, into two cells, the upper (distal) of
which is the new apical cell (b, ap. c), while the lower is now
distinguished as the sub apical or segmental cell (j. ap. c).
I'he sub-apical cell divides again horizontally, forming two
cells, the uppermost of which (c, 7 id ^) almost immediately
becomes divided by vertical planes into several cells (d, ;
the lower (c, i), int. 7 id^) remains undivided.
The sub*a])ical cell is the rudiment of an entire segment ;
the upi)ermost of the two cells into which it divides is the
rudiment of a node, the lower of an internode. The future
fate of the two is shown at once by the node dividing into
a horizontal plate of cells while the internode remains
unicellular.
Soon the cells of the new node begin to send out short
blunt processes arranged in a whorl : these increase in size,
undergo division, and form leaves (a — d, /^).
T'hese processes are continually being repeated ; the apical
cell is constantly producing new sub-apical cells, the sub-
aj)ical cells dividing each into a nodal and an internodal
cell ; and the nodal cell dividing into a horizontal plate of
cells and giving off leaves, while the internodal cell remains
undivided.
The special characters of the fully-formed parts of the
plant are due to the unequal growth of the new cells. The
nodal cells soon cease to grow and undergo but little altera-
tion (comp, fid ^ and nd^), whereas the internodes increase
immensely in length, being quite 3,000 times as long when
full-grown as when first separated from the sub-apical cell.
'The leaves also, at first mere blunt projections (a, /‘^), soon
increase sufficiently in length to arch over the growing point
and so form the characteristic terminal bud : gradually they
open out and assume the normal position, their successors
of the next younger whorl having in the meantime developed
XX
MULTIPLICATION OF NUCLEUS
211
sufficiently to take their place as protectors of the growing
point.
The miiltiniicleate condition of the adult internodes is
also a result of gradual change. In its young condition an
internodal cell has a single rounded nucleus (a, int. int,
7 id'% but by the time it is about as long as broad the nucleus
has begun to divide (d, int. ; c, int. nd^), and when the
length of the cell is eqiiiU to about twice its breadth, the
nucleus has broken up into numerous fragments (c, int. nd ^ ,
D, int. nd-), many of them still in active (amitotic) division.
This repeated fission of the nucleus reminds us of what
was found to occur in Opalina (p. 123).
Thus the growth of Nitella like that of Penicillium (p.
190), is apical : new cells arise only in the terminal bud,
and, after the first formation of nodes, internodes, and
leaves, the only change undergone by these parts is an in-
crease in size accomi)anicd by a limited differentiation of
character.
A shoot arises by one of the cells in a node sending
off a projection distal to a leaf, i.e., in an axil : the process
separates from the parent cell and takes on the characters of
the apical cell of the main stem, the structure of w^hich is in
this way exactly repeated by the shoot.
The leaves, unlike the branches, are strictly limited in
growth. At a very early period the apical cell of a leaf
becomes pointed and thick-walled (Fig. 44, e), and after this
no increase in the number of cells takes place.
The rhizoids also arise exclusively from nodal cells : they
consist of long filaments (Fig. 44, c), not unlike Mucor-
hyphae but occasionally divided by oblique septa into linear
aggregates of cells, and increase in length by apical growth.
The structure of the gonads is peculiar and somewhat
complicated.
p 2
212 MONOSTROMA. ULVA, AND NITELLA less.
As we have seen, the spermary (Fig. 44, G, spy) is a
globular, orange coloured body attached to a leaf by a short
stalk. Its wall is formed of eight pieces or shields, which
fit against one another by toothed edges, so that the entire
spermary may be compared to an orange in which an equa-
torial incision and two meridional incisions at right angles
to one another have been made through the rind, dividing
r 10, 46. — A, (hagranimatic vertical section of the spermary of Nitella,
showing the stalk {stk\ four of the eight shields each bearing on
its inner face a handle {hn), to which is attached a head-cell {hd ) : each
head-cell bears six secondary head-cells {hd*\ to each of which four
spermatic filaments {sp. f,) are attached.
B, one of the proximal shields {sit), with handle {hi), head-cell {hd),
secondary head-cells {hd'), and spermatic filaments {sp.f).
c, a single sperm.
nb n-, tnree sts^es in the development of the spermary.
(c, after Howes.)
it into eight triangular pieces. Strictly speaking, however,
only the four distal shields are triangular : the four proximal
ones have each its lower angle truncated by the insertion of
the stalk, so that they are actually four-sided.
Each shield (Fig. 46, a and b, sh) is a single concavo-
convex cell having on its inner surface numerous orange-
coloured chromatophores : owing to the disposition of these
XX
STRUCTURE OF SPERM ARY
213
on the inner surface only, the spermary appears to have a
colourless transparent outer layer — like an orange inclosed
in a close-fitting glass case.
Attached to the middle of the inner surface of each shield
is a cylindrical cell, the handle {/in)y which extends towards
the centre of the spermary, and, like the shield itself, con-
tains orange chromatophores. Each of the eight handles
bears a colourless head cell (hd), to which six secondary head
cells (Jid) are attached, and each of these latter bears four
delicate coiled filaments {sp. /) divided by septa into small
cells arranged end to end, and thus not unlike the hyphae of
a fungus. There are therefore nearly two hundred of these
spermatic filaments in each spermary, coiled uj) in its interior
like a tangled mass of white cotton.
The cells of which the filaments are composed have at
first the ordinary character, but as the spermary arrives at
maturity there is produced in each a single sperm (c), having
the form of a spirally-coiled thread, thicker at one end than
the other, and bearing at its thin end two long flagella. In
all probability the sperm proper, /.<?., the spirally coiled body,
is formed from the nucleus of the cell, the flagella from its
protoplasm. As each of the 200 spermatic filaments con-
sists of from 100 to 200 cells, a single spermary gives rise
to between 20,000 and 40,000 sperms.
When the sperms are formed the shields separate
from one another and the spermatic filaments protrude
between them like cotton from a pod : the sperms then
escape from the containing cells and swim freely in the
water.
The ovary (Fig. 44, G, ovy^ and Fig. 47 a) is ovoidal in
form, attached to the leaf by a short stalk (stk\ and ter-
minated distally by a little chimney-like elevation or crown
(cr). It is marked externally by spiral grooves which can be
214
MONOSTr<OMA, ULVA, AND NITELLA
LESS.
traced into the crown, and in young specimens its interior is
readily seen to be occupied by a large opaque mass (ov).
Sections show that this central body is the ovjim, a large cell
very rich in starch : it is connected with the unicellular stalk
by a small cell from which spring five spirally-arranged
cells (^/. c) ; these coil round the ovum and their free ends
— each divided by septa into two small cells — project at the
distal end of the organ and form the crown, enclosing a
Fl(’.. 47. — A, vertical section of tlic ovary of Nitclla, showinj; the
stalk (.v//’), small node {fid) from which spring the five S[)irally-twisted
cells (v/>. r), each ending in one of the tvvo-celleii sections of the crown
{i r). 'Fhe ovum contains starch grains, and is represented as trans-
parent, the spiral cells being seen through it.
Bh surface view, and section of a very young ovary ; later
stage in vertical section : id, still later stage, smface view, with the
oviun seen through the transparent spiral cells. Letters as in A, except
.V, small colls formed by division from the base of the ovum. (B--B'*
after Sachs.)
narrow canal which places the distal end of the ovum in free
communication with the surrounding water.
We saw how the various parts of the fully formed plant —
nodal, and internodal cells, leaves, and rhizoids — were all
formed by the modification of similar cells produced in the
apical bud. It is interesting to find that the same is true of
the diverse parts of the reproductive organs.
The spermary arises as a single stalked globular cell which
XX
DEVELOPMENT i)V (iONADS
215
becomes divided into eight octants (Fig. 46, d^). Each of
these then divides tangentially (/>. parallel to the surface
of the sphere) into two cells (d-), the inner of which divides
again (d‘^),so that each octant is now composed of three cells.
Of these the outermost forms the shield, the middle the
handle, and the inner the head-cell : from fhe latter the
secondary head-cells and spermatic filaments arc produced
by budding. The entire spermary appears to be a modified
leaflet.
The ovary also arises as a single cell, but soon divides and
becomes diflerentiated into an axial row of three cells (Fig.
47, B-, ozf, ndy stk) surrounded by five others {sp> c) which arise
as buds from the middle cell of the axial row (zid) and are
at first knob-like and upright (u^). The uppermost or distal
cell of the axial row becomes the ovum (b-^ b^, ov), the
others the stalk {stk) and intermediate cells {zid^ x) : the five
surrounding cells elongate, and as they do so acejuire a spiral
twist which becomes closer and closer as growth proceeds
(compare — b^, and Fig. 44, G, ozy). At the same time the
distal end of each develops tw'o septa (b*^) and, projecting
beyond the level of the ovum, forms with its fellows the
chimney or crown (cr) of the ovary. There is every reason
to believe that the entire ovary is a highly-modified shoot :
the stalk representing an internode, the cell nd a node, the
spiral cells leaves, and the ovum an apical cell.
Thus while the ciliate Infusoria and Caulerpa furnish ex-
amples of cell-differentiation without cell-multiplication, and
Spirogyra of cell-multiplication without cell-differentiation,
Nitella is a simple example of an organism in w^hich com-
plexity is obtained by the two processes going on hand in
hand. It is a solid aggregate, the constituent cells of which
are so arranged as to produce a well-defined external form,
216 MONOSTROMA, ULVA, AND NITELLA less.
while some of them undergo a more or less striking differen-
tiation according to the position they have to occupy, and
the function they have to perform.
Fig. 48. -—Knibryt) of Chara, an ally of Nitella, showiii}; the ovary
{cyvy), from the oosperm in which the embryo has sprung : the two
nodes (wr/), apical cell {ap. r), rhizoids {r/t), and leaves (/) of the
embryo : and the rudiment of the leafy plant (shaded) ending in the
characteristic terminal bud {term. bud). (After Howes, slightly altered. )
Impregnation takes place in the same manner as in
Vaucheria (p. 173). A sperm makes its way down the
canal in the chimney-like crown of cells terminating the
ovary, and conjugates with the ovum converting it into an
oosperm.
After impregnation the ovary, with the contained oosj^erm,
becomes detached and falls to the bottom, where, after a
XX
GERMINATION
217
period of rest, it germinates. The process begins by the
division of the oosperm into two cells, a small one nearest
the crown and composed almost wholly of protoplasm, and
a larger one full of starch granules. The larger cell serves
simply as a store of nutriment to the growing plant which
is itself developed exclusively from the small cell. The
latter divides into two cells one of which grows downwards
as a root-fibre, the other upwards as a shoot, consisting at
first of a single row of cells (Fig. 48). Soon two nodes {nd)
are formed on the filament, or embryo^ from the lower of
which rhizoids (rh) proceed, while the upper gives rise to a
few leaves (/), and to a small process which is at first uni*
cellular, but, behaving like an apical cell of Nitella, soon
becomes a terminal bud (term, bud) and grows into the
adult plant.
It will be seen that the development of Nitella is remark-
able for the facts that the adult plant is not formed directly
from the oosperm but that the latter gives rise to an embryo,
quite different from the adult in structure, and that, from
the embryo, the adult is finally developed as a lateral bud.
LESSON XXI
HYDRA.
have seen that with plants, !)oth F ungi and Algae, the
next stage of morphological differentiation after the simple
unicellular or non-ccllular organism is the linear aggregate .
Among animals there are no forms known to exist in this
stage, but coming immediately above the highest unicellular
animals, such as the ciliate Infusoria, we have true solid
aggregates. The characters of one of the simplest of these
and the fundamental way in which it differs from the plants
described in the two previous lessons will be made clear by
a study of one of the little organisms known as “ fresh-water
polypes ” and placed under the genus Hydra.
Although far from uncommon in pond- water, Hydra is not
always easy to find, being rarely abundant and by no means
conspicuous. In looking for it the best plan is to fill either
a clear glass bottle or beaker or a white saucer wath w^eeds
and water from a })ond and to let it remain undisturbed for
a few minutes. If the gathering is successful there will be
seen adhering to the sides of the glass, the bottom of the
saucer, or the weeds, little white, tawny, or green bodies,
about as thick as fine sewing cotton, and 2 — 6 mm. in
length. They adhere pretty firmly by one end, and examin-
Fig. 49. — Hydra.
A, Two living specimens of //. viridis attached to a bit of weed.
The larger specimen is fully expanded, and shows the elongated body
ending distally in the hypostome Khyp\ surrounded by tentacles (/), and
three buds {h(d, bd^., bd'"^) in different stages of development : a small
water-flea {a) has been captured by one tentacle. The smaller specimen
(to the right and above) is in a state of complete retraction, the tentacles
(/) appearing like papillae.
B, H fusca, showing the mouth {mth) at the end of the hypostome
{hyp\ the circlet of tentacles (/), two sperraaries {spy), and an ovary
{cvy).
c, a Hydra creeping on a flat surface by looping movements.
D, a specimen crawling on its tentacles.
(c and D after W, Marshall.)
220
HYDRA
LESS.
ation with a pocket lens shows that from the free extremity
a number of very delicate filaments, barely visible to the
naked eye, are given off.
Under the low power of a compound microscope a Hydra
(Fig. 49, b) is seen to have a cylindrical body attached by a
flattened base to a weed or other aquatic object, and bearing
at its opposite or distal end a conical structure, the hyposiome
{hyp\ at the apex of which is a circular aperture, the mouth
(nith). At the junction of the hypostome with the body
proper are given off from .six to eight long delicate ten-
tacles (t) arranged in a circlet or whorl. A longitudinal
section shows that the body is hollow, containing a .spacious
cavity, the enteron (Fig. 50, a, ent, cav\ which communicates
with the surrounding water by the mouth. The tentacles are
also hollow, their cavities communicating with the enteron.
There are three kinds of Hydra commonly found ; one,
H, vulgaris^ is colourless or nearly so ; another, II. fusca^ is
of a i^inkish-yellow or brown colour ; the third, H. viridis^ is
bright green. In the two latter it is quite evident, even
under a low power, that the colour is in the inner parts of
the body-wall, the outside of which is formed by a transparent
colourless layer (Fig. 49, a, b).
It is quite easy to keep a Hydra under observation on the
stage of the microscope for a considerable time by placing it
in a watch-glass or shallow cell ’’ with weeds, &c., and in
this way its habits can be very profitably studied.
It will be noticec^ in the first place, that its form is
continually changing. At one time (Fig. 49, a, left-hand
figure) it extends itself until its length is fully fifteen times its
diameter and the tentacles appear like long delicate filaments:
at another time (right-hand figure) it contracts itself into an
almost globular mass, the tentacles then appearing like little
blunt knobs.
xxr
MOVEMENTS
221
Besides these movements of contraction and expansion,
Hydra is able to move slowly from place to place. This it
usually does after the manner of a looping caterpillar (Fig.
49, c) : the body is bent round until the distal end touches
the surface : then the base is detached and moved nearer the
distal end, which is again moved forward, and so on. It has
also been observed to crawl like a cuttle fish (i>) by means of
its tentacles, the body being kept nearly vertical.
It is also possible to watch a Hydra feed. It is a very
voracious creature, and to see it catch and devour its prey is
a curious and interesting sight. In the water in which it
lives are always to be found numbers of water-fleas,” minute
animals .from about a millimetre downwards in length,
belonging to the class Crus/acea^ a group which includes
lobsters, crabs, shrimps, &c.
Water-fleas swim very rapidly, and occasionally one may be
seen to come in contact with a Hydra^s tentacle. Instantly
its hitherto active movements stop dead, and it remains
adhering in an apparently mysterious manner to the tentacle.
If the Hydra is not hungry it usually liberates its prey after
a time, and the water-flea may then be seen to drop through
the water like a stone for a short distance, but finally to
expand its limbs and swim off. If however the Hydra has
not eaten recently it gradually contracts the tentacle until
the prey is brought near the mouth, the other tentacles being
also used to aid in the proce.ss. The water-flea is thus forced
against the apex of the hypostome, the mouth expands
widely and seizes it, and it is finally passed down into the
digestive cavity. Hydrae can often be seen with their bodies
bulged out in one or more places by recently swallowed
water-fleas.
The precise structure of Hydra is best made out by cutting
222
riVDRA
LESS.. XXI
it into a series of extremely thin sections and examining
them under a high power. The appearance presented by a
vertical section through the long axis of the body is shown
in Fig. 50.
The whole animal is seen to be built up of cells, each
consisting of protoplasm with a large nucleus (b, c, nu\ and
with or without vacuoles. As in the case of most animal
cells, there is no cell-wall. Hydra is therefore a solid aggre-
gate: but the way in which its constituent cells are arranged
is highly characteristic and distinguishes it at once from a
plant.
The essential feature in the arrangement of the cells is
that they are disposed in two layers round the central
digestive cavity or enteron (a, eni. caif) and the cavities of
tentacles cav'). So that the wall of the body is formed
throughout of an outer layer of cells, the ectoderm {ect\ and
of an inner layer, tlie endoderm (end), which bounds the
enteric cavity. Between the two layers is a delicate trans-
parent membrane, the mesi\^liea, or supporting lamella (tnsgl).
transverse section shows that the cells in both layers are
arranged radially (b).
Thus Hydra is a two-layered or diploblastic animal, and
may be compiired to a chimney built of two layers of radially
arranged bricks with a space between the layers filled with
mortar or concrete.
Accurate examination of thin sections, and of specimens
teased out or torn into minute fragments with needles, shows
that the structure is really much more complicated than the
foregoing brief description would indicate.
'I'he ectoderm cells are of two kinds. The first and most
obvious (b, ect and c), are large cells of a conical form, the
bases of the cones being external, their apices internal. Spaces
Fig. — Hydra,
A, A nematocyst contained in its cnidot>!ast {rnh), showing the coiled
filament and the cnidocil {end),
B, Tile same after extrusion of the thread, showing the larger and
smaller barbs at the base of the thread, nu, the nucleus of the
cnidoblast.
c, A cnidoblast, with its contained nematocyst, connected with one
of the ])rocesses of a nerve cell {nv, c).
(After Schneider.)
226
HYDRA
LESS.
degree of contractility is assigned to the muscle-processes
while the cells themselves are eminently irritable, the slightest
stimulus applied to them being followed by an immediate
contraction of the whole body.
Imbedded in some of the large ectoderm cells arc found
clear, oval sacs (a and b, //A*), with very well-defined walls
and called nematocysts. Both in the living specimen and in
sections they ordinarily present the appearance shown in
Fig. 50, u. ntc^ and Fig. 51 a, but are frequently met with
in the condition shown in Fig. 50 e, and Fig. 5 1 b : that
is, with a short conical tube protruding from the mouth of
the sac, armed near its distal end with three recurved
barbs besides several similar processes of smaller size,
and giving rise distally to a long, delicate, fle.xible fila-
ment.
Accurate examination of the nematocysts shows that the
structure of these curious bodies is as follows. Each con-
sists of a tough sac (Fig. 51, a), one end of w'hich is turned
in as a hollow pouch : the free end of the latter is continued
into a hollow coiled filament, and from its inner surface
project the barbs. The whole space between the wall of
the sac and the contained pouch and thread is tensely filled
with fluid. When pressure is brought to bear on the outside
of the sac the whole apparatus goes off like a harpoon-gun
(h), the compression of the fluid forcing out first the barbed
pouch and then the filament, until finally both are turned
inside out.
It is by means of the nematocysts — the resemblance of
which to the trichocysts of Paramcecium (p. 1 13) should be
noted — that the Hydra is enabled to paralyze its prey. Prob-
ably some specific poison is formed and ejected into the
wound with the thread : in the larger members of the group
to which Hydra belongs, suchasjelly-fishes, the nematocysts
XXI
NEMATOCYSTS
227
produce an effect on the human skin quite like the sting of
a nettle.
The nematocysts are formed in special interstitial cells
called cnidoblasts (Fig. 50, u, cnbl and Fig. 51), and are thus
in the first instance at a distance from the surface. But the
cnidoblasts migrate outwards, and so come to lie quite
superficially either in or between the large ectoderm cells.
On its free surface the cnidoblast is produced into a delicate
pointed process, the cnidocil or “ trigger-hair (end). In all
probability the slightest touch of the cnidocil causes con-
traction of the cnidoblast, and the nematocyst, thus com-
pressed, instantly explodes. '
'Nematocysts are found in the distal part of the body, but
are absent from the foot or proximal end, where also there
are no interstitial cells. They are especially abundant in the
tentacles, on the knob-like elevations of which — due to little
heaps of interstitial cells — they are found in great numbers.
Amongst these occur small nematocysts with short threads
and devoid of barbs (Fig. 50, A, ntc and f).
There are sometimes found in connection with the cnido-
blasts small irregular cells with large nuclei : they are called
nerve-cells (Fig. 51, c, nv. c), and constitute a rudimentary
nervous system, the nature of which will be more con-
veniently discussed in the next lesson (p. 242).
The ectoderm cells of the foot differ from those of the rest
of the body in being very granular (Fig. 50 a). The
granules are probably the material of the adhesive substance
by which the Hydra fixes itself, and are to be looked upon as
products of destructive metabolisms t.e. as being formed by
conversion of the protoplasm in something the same way as
starch granules (p. 33). This process of formation in a cell
of a definite product which accumulates and is finally dis-
charged at the free surface of the cell is called secretion,
Q 2
228
HVDRA
LESS.
and the cell performing the function is known as a gland-
cell.
The cndodcrm consists for the most part of large cells
whi(^h exceed in size those of the ectoderm, and are re-
markable for containing one or more vacuoles, sometimes
so large as to reduce the protoplasm to a thin su])erficial
layer containing the nucleus (Fig. 50, a and b, end). Then
again, their form is extremely variable, their free or inner
ends undergoing continual changes of form. This can be
easily made out by cutting transverse sections of a living
Hydra, when the endoderm cells are seen to send out long
blunt pseudopods {psd) into the digestive cavity, and now
and then to withdraw the pseudopods and send out from
one to three long delicate flagella {fl\ Thus the endoderm
cells of Hydra illustrate in a very instructive manner the
essential similarity of flagella and pseiidopods already re-
ferred to (p. 52). In the hypostome the endoderm is thrown
into longitudinal folds, so as to allow of the dilatation of
the mouth in swallowing.
Amongst the ordinary endodcrm-cells are found long
narrow cells of an extremely granular character. They are
specially abundant in the distal part of the body, beneath
the origins of the tentacles, and in the hypostome, but are
absent in the tentacles and in the foot. There is no doubt
that they are gland-cells, their secretion being a fluid used
to aid in the digestion of the food.
In Hydra viridis the endoderm-cells (d) contain chrorna-
tophores {(hr) coloured green by chlorophyll, which performs
the same function as in plants, so that in this species holozoic
is supplemented by holophytic nutrition. There is reason
for believing that the chromatophores are to be regarded as
symbiotic algae, like those found in connection with Radio-
XXI
DIGESTION
229
laria(p. 154). In H. fusca bodies resembling these chromato
phores are present, but are of an orange or brown colour, and
devoid of chlorophyll. Brown and black granules occurring
in the cells ( b) seem to be due in part to the degeneration of
the chromatophorcs, and in part to be products of excretion.
Muscle-processes exist in connection with the endoderm
cells, and they are said to take a transverse or circular
direction, />., at right angles to the similar processes of
the ectoderm cells.
When a water-flea or other minute organism is swallowed
by a Hydra, it undergoes a gradual process of disintegration.
The process is begun by a solution of the soft i)arts due to
the action of a digestive fluid secreted by the gland-cells of
the endoderm ; it is apparently completed by the endoderm
cells seizing minute particles with their t)seiidopods and
engulfing them quite after the manner of Amoebae. It is
often found that the protrusion of pseudopods during
digestion results in the almost complete obliteration of the
enteric cavity.
It would seem therefore that in Hydra the process of
digestion or solution of the food is to some extent at least
intra-celluiar^ />., takes place in the interior of the cells
themselves, as in Amoeba or Paramoecium : it is however
mainly extra-cellular or enteric is performed in a special
digestive cavity lined by cells.
The ectoderm cells do not take in food directly, but are
nourished entirely by diffusion from the endoderm. I'hus
the two layers have different functions : the ectoderm is pro-
tective and sensory ; it forms the external covering of the
animal, and receives impressions from without ; the endo-
derm, removed from direct communication with the outer
world, performs a nutrient function, its cells alone iiaving
the power of digesting food.
2y> HYDRA less.
The essential difference between digestion and assimilation
is here plainly seen : all the cells of Hydra assimilate, all
are constantly undergoing waste, and all must therefore form
new protoplasm to make good the loss. JUit it is the endo-
derm cells alone which can make use of raw or undigested
food : the ectoderm has to depend upon various products of
digestion received by osmosis from the endoderm.
It will be evident from the preceding description that
Hydra is comparable to a colony of Amoebae in which par-
ticular functions are made over to particular individuals —
just as in a civilized community the functions of baking and
butchering are assigned to certain members of the commu-
nity, and not performed by all. Hydra is therefore an ex-
ample of individuation: morphologically it is equivalent
to an indefinite number of unicellular organisms : but,
these acting in concert, some taking one duty and some
another, form, physiologically speaking, not a colony of
largely independent units, but a single multicellular in-
dividual.
Like many of the organisms which have come under
our notice, Hydra has two distinct methods of reproduction,
asexual and sexual.
Asexual multiplication takes place by a process of budding.
A little knob appears on the body (Fig. 49, a, bd^\ and is
found by sections to arise from a group of ectoderm cells ;
soon however it takes on the character of a hollow out-
pushing of the wall containing a prolongation of the enteron,
and made up of ectoderm, mesoglcea, and endoderm. (Fig.
50, A, bd^\ In the course of a few hours this prominence
enlarges greatly, and near its distal end six or eight hollow
buds appear arranged in a whorl (Fig. 49, a, bd ^ ; Fig. 50,
XXI
REPRODUCTION
231
A, These enlarge and take on the characters of ten-
tacles : a mouth is formed at the distal end of the bud,
which thus acquires the character of a small Hydra (Fig.
49, A, Finally the bud becomes constricted at its base,
separates from the parent, and begins an independent ex-
istence. Sometimes, however, several buds are produced at
one time, and each of these buds again before becoming
detached : in this way temporary colonies arc formed. Ilut
the buds always separate sooner or later, although they
frequently begin to feed while still attached.
It is a curious circumstance that Hydra can also be mul-.
tiplied by artificial division : the experiment has been tried
of cutting the living animal into pieces, each of which was
found to grow into a perfect individual.
As in Vaucheria and Nitella, the sexual organs or gonads
are of two kinds, spermaries and ovaries. Both are found
in the same individual. Hydra being, like the plants just
mentioned, hermaphrodite or monoecious*
The spermaries (Fig. 49, b, and Fig. 50, a, spy) are white
conical elevations situated near the distal end of the body :
as a rule not more than one or two are present at the same
time, but there may be as many as twenty. They are j)er-
fectly colourless, even in the green and brown species, being
obviously formed of ectoderm alone.
In the immature condition the spermary consists of a little
heap of interstitial cells covered by an investment of some-
what flattened cells formed by a modification of the ordinary
large cells of the ectoderm. When mature each of the small
internal cells becomes converted into a sperm (Fig. 50, g),
consisting of a small ovoid head formed from the nucleus of
the cell, and of a long vibratile tail formed from its proto-
plasm. By the rupture of the investing cells or wall of the
232
HYDRA
LESS.
speniiary the sperms are liberated and swim freely in the
water,
I'he ovaries (Fig. 49, b, and Fig. 50, a, ovy) are found
near the proximal end of the body, and vary in number from
one to eight. When ripe an ovary is larger than a spermary,
and of a hemispherical form. It begins, like the spermary,
as an aggregation of interstitial cells, so that in their earlier
stages the sex of the gonads is indeterminate. f 3 ut while
FiCr, 52. — A, Ovum of Hydra viridis^ showing pseudopods, nucleus
an<l numerous chromatophores and yolk spheres.
H, a single yolk sphere. (From IJalfuur after Kleinenl>erg. )
in the spermary each cell is converted into a sperm, in the
ovary one cell soon begins to grow faster than the rest
becomes amoeboid in form (Fig. 50, a, (n\ and Fig. 52, a),
sending out pseudopods amongst its companions and ingest-
ing the fragments into which they become broken up, thus
continually increasing in size at their expense. Ultimately
the ovary comes to consist of this single amoeboid (nmniy
and of a layer of superficial cells forming a capsule for it.
XXI DEVELOPMENT 233
As the ovum grows yolk-spheres (Fig. 52), small rounded
masses of proteid material, are formed in it, and in Hydra
viridis it also acquires green chromatophores.
When the ovary is ripe the ovum draws in its j)seudopods
and takes on a spherical form ; the investing layer then
bursts so as to lay bare the ovum and allow of the free access
to it of the hjicrms. One of the latter conjugates with the
ovum, producing an oosperm or unicellular embryo.
The oosperm divides into a number of cells, the outer-
most of which becomes changed into a hard shell or capsule.
1'he embryo, thus jirotected, falls to the bottom of the water,
and after a period of rest develops into a Hydra. As, how-
ever, there are certain abnormal features about the develoi>-
rncnt of this genus which cannot well be understood by the
beginner, it will not be described in detail, but the very
imjjortant series of changes by which the oosperm of a
multicellular animal becomes converted into the adult will
be considered in the next lesson.
LESSON XXII.
HYDROin POI.YPKS 'ROUGAINVILLEA, DIPHYES, AND PORPITA,
It was stated in the previous lesson (p. 231) that in a
budding Hydra the l)uds do not always become detached
at once, but may themselves bud while still in connection
with the parent, temporary colonies being thus produced.
Suppose this state of things to continue indefinitely : the
result would be a tree-like colony or compound organism
consisting of a stem with numerous branchlets each ending
in a Hydra-like zooid. Such a colony would bear much the
same relation to Hydra as Zoothamnium bears to Vorticella
(sfC p. 134).
As a matter of fact this is precisely what happens in a
great number of animals allied to Hydra and known by the
name of Zoophytes or Hy droid polypes*
Every one is familiar with the common Sertularians of the
sea-coast, often mistaken for sea-weeds: they are delicate,
much-branched, semi-transparent structures of a horny con-
sistency, the branches beset with little cups, from each of
which, during life, a Hydra-like body is protruded.
A very convenient genus of hydroid polypes for our pur-
pose is Bottgatfivil/ea^ found in the form of little tufts a few
centimetres long attached to rocks and other submarine
A
Fig. 53. — Bougaimfillea ramosa.
A, a complete living colony of the natural size, showing the branched
stem and root-like organ of attachment.
B, a portion of the same magnified, showing the branched stem bear-
ing hydranths {Ayd) and medusae (med), one of the latter nearly mature,
the others undeveloped : each hydranth has a circlet of tentacles (/)
surrounding a hypostome and contains an enteric cavity {cfd, cati)
continuous with a narrow canal {ent. cav') in the stem. The stem is
covered by a cuticle (t «).
c, a medusa after lil>eration from the colony, showing the l>ell with
tentacles (/), velum (?/), manubrium radial {rad, c) and circular
(«>. r) canals, and eye-spots (ac). (After Allman.)
236 HYDROID POLYPES less.
objects. Fig. 53, a, shows a colony of the natural size, b a
part of it magnified : it consists of a much-branched stem of
a yellowish colour attached by root-like fibres to the support.
The bran^'hes terminate in little Hydra-like bodies called
hydranths ( b , hyd\ each with a hypostome {hyp) and circlet of
tentacles (/). Lateral branchlets bear bell-shaped structures
or 7tiedus(V {med) : these will be considered presently.
Sections show that the hydranths have essentially the
structure of a Hydra, consisting of a double layer of cells
— ectoderm and endoderm - separated by a supporting
lamella or mesogkea, and enclosing a digestive cavity {ent,
cav) which opens externally by a mouth placed at the
summit of the hypostome.
The tentac'les, however, differ from those of Hydra in two
imiiortant respects. In the first place they are solid : the
endoderm instead of forming a lining to a prolongation of
tile enteron, consists (Fig. 55, end,) of a single axial row of
large cells with thick cell-walls and vacuolated protoplasm.
Then in the position of the muscle-processes of Hydra there
is a layer of spindle-sha]ied fibres (///./), many times
longer than broad, and provided each with a nucleus. Such
musch’fibres are obviously cells greatly extended in length, so
that the ectoderm cell of Hydra with its continuous muscle-
ptocess is here represented by an ectoderm cell with an
adjacent muscle-c^//. We thus get a partial intermediate
layer of cells between the ectoderm and endoderm in
addition to the gelatinous mesogloea, and so, while a hydroid
[X)lyp is, like Hydra, diploblasiic {^, 222), it shows a tendency
towards the assumption of a three-layered or triploblastic
condition.
The stem is formed of the same layers and contains a
cavity {ent, ca%^') continuous with those of the hydranths,
and thus the structure of a hydroid polype is, so far, simply
xxn
STRUCTURE OF COLONY
237
that of a Hydra in which the process of budding has
gone on to an indefinite extent and without separation of
the buds.
Fir,. 54. — Portion of the tentacle of a Zoojjhytc (Eucopclla),
In the lower part of the figure are seen the ectoderm cells [ect) with
the nematoepts ^ntc). In the middle part the ectoderm is removed, and
the muscle-nhres {vi.f) and nerve-cells {nv. c) are exposed. Jn the
upper part the muscular and nervous layer is removed, and parts of two
endoderm cells {end) are shown ; ///r, nucleus.
(From Parker and Ilaswell, after von I^endcnfeld.)
There is however an additional layer added in the stem
for protective and strengthening purposes. It is evident
that a colony of the size shown in Fig. 53, a, would, if formed
238 HYDROID POLYPES less.
only of soft ectodermal and endodcrmal cells, be so weak as
to he hardly able to bear its own weight even in water. To
remedy this a layer of transparent, yellowish substance of
horny ('onsistency, called the cuticle, is developed outside
the ectoderm of the stem, extending on to the branches and
only stopping at the bases of the hydranths and medusae.
It is this layer which, when the organism dies and decays,
is left as a semi-transparent branched structure resembling
the living colony in all but the absence of hydranths and
medusie. 'The cuticle is therefore a supporting organ or
skeleton, not, like our own bones, formed in the interior
of the body {endoskeleton), but like the shell of a crab
or lobster lying altogether outside the soft parts {exo-
skeleton).
As to the mode of formation of the cuticle : — we saw that
many organisms, such as Amoeba and Heematococcus, form,
on entering into the resting condition, a cyst or cell-wall, by
secreting or separating from the surface of their protoplasm
a succession of layers either of cellulose or of a transparent
horn-like substance. But Amoeba and Haematococcus are
unicellular, and are therefore free to form this protective
layer at all piirts of their surface. The ectoderm cells of
Bougainvillea on the other hand are in close contact with
their neighbours on all sides and with the mesogloea at their
inner ends, so that it is not surprising to find the secretion
of skeletal substance taking place only at their outer ends.
As the process takes place simultaneously in adjacent cells,
the result is a continuous layer common to the whole
ectoderm instead of a capsule to each individual cell. It is
to an exoskeletal structure formed in this way, i.e. by the
secretion of successive layers from the free faces of adjacent
cells, that the name cuticle is in strictness applied in multi-
cellular organisms.
XXII
STRUCTURE OF A MEDUSA
239
The medusnc (u, med, and c), mentioned above as occur-
ring on lateral branches of the colony, are found in various
stages of development, the younger ones having a nearly
globular shape, while when fully formed each resembles a
bell attached by its handle to one of the branches of the
colony and having a clapper in its interior. AVhen quite
mature the medusae become detached and swim off as little
jelly-fishes (c).
The structure of a medusa must now be described in
some detail. The bell or umbrella (c) is formed of a gela-
tinous substance (Fig. 55, i), msgl) covered on both its inner
surface or sub-umbrella and on its outer surface or ex-umbrella
by a thin layer of delicate cells {ect\ The clajiper-like
organ or manubrium (Fig. 53, c and Fig. 55 d and o', mnb)
is formed of two layers of cells, precisely resembling the
ectoderm and endoderm of Hydra, and sejiarated by a thin
mesogkjca; it is hollow, its cavity (Fig. 55, i), ent, cav) open-
ing below, i,e, at its distal or free end, by a rounded aperture,
the mouth {mth)^ used by the medusa for the ingestion of
food. At its upper (attached or ])roximal) end’ the ('avity of
the manubrium is continued into four narrow, radial canals
(Fig. 53, c, rad. r, and Fig. 54, d and \> rad) which extend
through the gelatinous substance of the umbrella at efjual
distances from one another, like four meridians, and finally
open into a circular canal (cir. c) which runs round the edge
of the umbrella. The whole system of canals is lined by a
layer of cells (Fig. 55, d and d', end) continuous with the
inner layer or endoderm of the manubrium ; and extending
from one canal to another in the gelatinous substance of the
umbrella is a delicate sheet of cells, the endoderm-laniella
(d', end. la).
From the edge of the umbrella four pairs of tentacles
(Fig. 53, c and Fig, 55, d, /) are given off, one pair corres-
240
HYDROID POLYPES
ponding to eacli radial canal, and close to the base of each
tentacle is a little speck of pigment (Eig. 53, oc\ the ocellus
or eye-spot, [.astly, the margin of the umbrella is continued
inwards into a narrow circular shelf, the velinn {v).
At first sight there appears to be very little resemblance
between a medusa and a hydranth, but it is really quite
easy to derive the one form from the other.
Su])jx)se a simple polype or Hydra-like body with four
tentacles (Fig. 55, a, a') to have the region from which the
tentacles spring pulled out so as to form a hollow, trans-
versely extended disc (h). Next, suppose this disc to become
bent into the form of a cup with its concavity towards the
hypostome, and to undergo a great thickening of its meso-
gkea, A form would be produced like c, />. a medusa-like
body with umbrella and manubrium, but with a continuous
cavity (c', cut. ca 7 >) in the thickness of the umbrella instead of
four radial canals. Finally, suppose the inner and outer walls
of this cavity to grow towards one another and meet, thus
obliterating the cavity, except along four narrow radial areas
(i>, rad) and a circular area near the edge of the umbrella
(l), dr. c). This would result in the substitution for the
continuous cavity of four radial canals opening on the one
hand into a circular canal and on the other into the cavity
of the manubrium {ent. cav)y and connected with one another
by a membrane — the endoderm-lamella {cud. la ) — indi-
cating the former extension of the cavity.
It follows from this that the inner and outer layers of the
manubrium are respectively endoderm and ec toderm : that
the gelatinous tissue of the umbrella is an immensely
thickened mesoglcea : that the layer of cells covering both
inner and outer surfaces of the umbrella is ectodermal : and
that the layer of cells lining the system of canals, together
with the endoderm-lamella, is endodermal.
Fig. 55. — Diagrams illustrating the derivation of the medusa from
the hy<lranth. In the whole series of figures the cctodenn {ec/) is dotted,
the endcxierm (enJ) striated, and the mesogloea black.
A, longitudinal section of a simple polype, showing the tubular body
with enteric cavity {enf. cav), hypostome {hyp)^ fnouth (wM), and
tentacles (/).
R
242
HYDROID POLYPES
LESS.
a', transverse section of the same through the plane a b,
B, the tentacular region is extended into a hollow disc,
c, the tentacular region has been further extended and bent into a
bell-likc form, the enteric cavity being continued into the umbrella
(ent, cav ') : the hypostome now forms a manubrium {mnb).
c\ transverse section of the same through the plane a by showing the
continuous cavity {ent. cav^) in the umbrella.
D, fully formed medusa : the cavity in the umbrella is reduced to the
radiating ^rad) and circular {cir, c) canals, the velum [v) is formed, and
a double nerve-ring {nVy nv*\ is produced from the ectoderm.
d', transverse section of the same through the plane a b, showing the
four radiating canals (rad) united by the endoderm-lamella (atd. /a),
produced by partial obliteration of the continuous cavity efU, cav' in C'.
Thus the medusa and the hydranth are similarly con-
structed or homologous structures, and the hydroid colony,
like Zoothamnium (p. 136), is dimorphic, bearing zooids of
two kinds.
Sooner or later the meduste separate from the hydioid
colony and begin a free existence. Under these circui
stances the rhythmical contraction — contraction taking
place at regular intervals — of the muscles of the umbrella
causes an alternate contraction and expansion of the whole
organ, so that water is alternately pumped out of and drawn
into it. The obvious result of this is that the medusa is pro-
pelled through the water by a series of jerks. The movement
is performed by means of the muscle-processes and muscle-
fibres of the sub-umbrella and velum, both of which diffei
from the similar structures in the hydranth in exhibiting a
delicate transverse striation (Fig. 57).
There is still another important matter in the structure of
the medusa which has not been referred to. At the junction
of the velum with the edge of the bell there lies, imme-
diately beneath the ectoderm, a layer of peculiar branched
cells (Fig. 56, B, «. €)y containing large nuclei and produced
into long fibre-like processes. These nerve-cells (see p. 227)
xxn
NERVOUS SYSTEM
243
are so disposed as to form a double ring round the margin
of the bell, one ring (Fig. 55, d, nv) being immetliately
above, the other {nv') immediately below the insertion of the
velum. An irregular network of similar cells and fibres o(’( iirs
on the inner or concave face of the umbrella, between the
ectoderm and the layer of muscle-fibres. The wliole eonsti-
Fig. 56. — A, Muscle fibres from the inner face of the bell of the
medusa of a hydroid polype {Eucopclla campanuland)^ showing nucleus
and transverse strialion.
B, portion of the nerve- ring of the same, showing two large nerve-
cells (//. c) and muscle- fibres (w. c) on either side. (After von Lcn-
denfeld. )
tutes the fiervous system of the medusa ; the double nerve-ring
is the central^ the network the peripheral nervous system.
Some of the processes of the niTve-cells are connected
with ordinary ectoderm-cells, \\hi( h thus as it were connect
the nervous system with the external world : others, in sinnc
instances at least, are probably directly connected with
muscle-fibres.
We thus see that while the manubrium of a medusa has
the same simple structure as a hydranth, or what comes to
R 2
244
IIYDROID POLYPES
LESS.
the same thing, as a Hydra, the umbrella has undergone a very
remarkable differentiation of its tissues. Its ordinary ecto-
derm cells, instead of being large and eminently contractile,
form little more than a thin cellular skin or epithelium over
the gelatinous mesogkjca: they have largely given up the
function of contractility to the muscle processes or fibres,
and have taken on the functions of a protective and sensitive
layer.
Similarly the function of automatism, possessed by the
whole body of Hydra, is made over to the group of specially
modified ectodermal cells which constitute the central
nervous system. If a Hydra is cut into any number of
pieces, each of them is able to jicrform the ordinary move-
ments of expansion and contraction, but if the nerve-ring
of a medusa is removed by cutting away the edge of the
umbrella, the rhythmical swimming movements stop dead :
the bell is in fact permanently paralysed.
It is not, however, rendered incapable of movement, for
a sharp pinch, i.e. an external stimulus, causes a single con-
traction, showing that the muscles still retain their irritability.
But no movement takes place without such external stimulus,
each stimulus giving rise infallibly to one single contraction :
the power possessed by the entire animal of independently
originating movement, i,e. of supplying its own stimuli, is
lost with the central nervous system.
Another instance of morphological and physiological
differentiation is furnished by the pigment spots or ocelli
53» Cj situated at the bases of the tentacles. They
consist of groups of ectoderm cells in which are deposited
granules of deep red pigment. Their function is proved by
the following experiment.
If a number of medusje are placed in a glass vessel of
water in a dark room, and a beam of light from a lantern is
xxn
GONADS
245
allowed to pass through the water, the animals are all found
to crowd into the beam, thus being obviously sensitive to and
attracted by light. If however the ocelli are removed this
is no longer the case : the meduscc do not make for the
beam of light, and are incapable of distinguishing light from
darkness. 'I'he ocelli are therefore organs of sight.
In Zoothamnium we saw that the two forms of zooid were
resj^ectively nutritive and reproductive in function, the re-
productive zooids becoming d(!tached and swimming off to
found a new colony elsewhere (p. 136).
This is also the case with Bougainvillea : the hydranths
are purely nutritive zooids, the meduspc, although capable of
feeding, are specially distinguished as reproductive zooids.
Tlie gonads are found in the walls of the manubrium,
between the ectoderm and endoderm, some medusae pro-
ducing ovaries, others spermaries only, 'i'hus while Hydra
is monceciotis^ both male and female gonads occurring in the
same individual, Bougainvillea is dicecious, certain individuals
producing only male, others only female products.
In some Hydroids it has been found that the sexual cells
from which the ova and sperms are developed do not originate
in the manubrium of a medusa, but arise in the first in-
stance from the ectoderm of the stem 6 the hydroid
colony, afterwards migrating, while still small and im-
mature, to their permanent situation where they undergo
their final development. In Bougainvillea, however, the
reproductive products are said to originate in the manubrium.
The medusae, when mature, become detached and swim
away from the hydroid colony. The sperms of the males
are shed into the w^ater and carried to the ovaries of the
females, where they fertilize the ova, converting them, as
usual, into oosperms.
246
IIVDROID POLYPES
LESS.
The changes by which the oosperm or unicellular embryo
of a hydroid polype is converted into the adult are very
remarkable.
1'he process is begun by the oosperm, still enclosed
within the body of the parent (Fig. 57, a), undergoing
binary fission, so that a two-celled embryo is formed (b).
lilach of the two cells again divides (c), and the process is
repeated, the embryo consisting successively of 2, 4, 8, 16,
32, &c., cells, until a solid globular mass of small cells is
produced (i), k) by the repeated division of the one large
cell which forms the starting-point of the series. The embryo
in this stage has been compared to a mulberry, and is called
the momhi or polyplasi.
So far all the cells of the polyplast are alike — globular
nucleated masses of protoplasm squeezed into a polyhedral
form by mutual pressure. But before long the cells lying
next the surface alter their form, becoming cylindrical, with
their long axes disposed radially (f). In this way a superficial
layer of cells, or ectoderm^ is differentiated from an internal
mass, or endoderm.
The embryo now assumes an elongated form (g) and
begins to exhibit slow, worm-like movements, finally escaping
from the parent and beginning a free existence (h). The
ectoderm cells are now found to be ciliated, and before long
a cavity appears in the previously solid mass of endoderm
cells : this is the first appearance of the enteron or digestive
cavity. In this stage the embryo is called a planula : it
swims slowly through the water by means of its cilia, the
broader end being directed forwards in progression. It then
loses its cilia and settles down on a rock, shell, sea-weed, or
other submarine object, assuming a vertical position with its
broader end fixed to the support (i).
The attached or proximal end widens into a disc of attach-
Fig. 57. — Stages in the development of two hydroid polypes, LaO'
medea Jiextiosa (a-h) and Etidendrium yamosuj?t (l-M).
A, oosperm.
B, two-celled, and C, four-celled stage.
D, E, polyplast.
F, G, formation of planula by differentiation of ectoderm and
endoderm.
In A-G the embryo is embedded in the maternal tissues.
H, free swimming planula, showing ciliated ectoderm, and endoderm
enclosing a narrow enteric cavity.
r, planula, after loss of its cilia, about to affix itself,
K, the same after fixation,
I. , Hydra-like stage, still enclosed in cuticle.
M, the same after rupture of the cuticle and liberation of the tentacles,
(After Allman.)
distal end, and a thin cuticle is secreted from the whole
surface of the ectoderm (k). From the dilated portion
248 HYDROTD POLYPES less.
short buds arise in a circle : these are the rudiments of the
tentacles : the narrow portion beyond their origin becomes
the hypostome (l). Soon the cuticle covering the distal end
is ru[)tured so as to set free the growing tentacles (m) : an
aperture, the mouth, is formed at the end of the hypostome,
and the young hydroid has very much the appearance of a
Hydra with a broad disc! of attachment, and with a cuticle
covering the greater part of the body.
ICxtensive budding next takes place, the result being the
formation of the ordinary hydroid colony.
d'hus from the oosperm or impregnated egg-cell of the
medusa the hydroid colony arises, while the medusa is
produced by budding from the hydroid colony. We have
what is called an alternation of generations^ the asexual genera-
tion or agamohium (hydroid colony) giving rise by budding
to the sexual generation ox gxmobium (medusa), which in its
turn produces the agamobium by a se.xual process, i.e. by
the conjugation of ovum and sperm.
'Pwo other Hydroids must be briefly referred to in con-
cluding the present lesson.
Floating on the surfrcc of the ocean in many parts of the
world is found a beautiful transparent organism called
Dip lives. It consists of a long, slender stem (Fig. 58, a, a\
at one end of which are attached two structures called
swimming-bells (///, w) in form something like the bowl of a
German pipe, while all along the stem spring at intervals
groups of structures (<?), one of which is showm on an
enlarged scale at b.
Each group contains, first, a tubular structure (b, n) with
an expanded, trumj>et-like mouth, through which food is
taken ; this is clearly a hydranth. From the base of the
hydranth proceeds a single^ long, branched tentacle or
XXII
DIPHYES AND PORPITA
249
“grappling-line (/), abundantly provided with nematocysts.
Springing from the stem near the base of the hydranth is a
body called a medusoid very like a sort of imperf(‘('t
medusa, and like it, containing gonads. Lastly, enclosing all
these structures, much as the white petaloid bract of the
common Arum-lily encloses the flower-stalk, is a delicate
folded membranous [)late (/), to which the name bract,
borrowed from botany, is applied. 'The whole organism is
propelled through the water by the rhythmical contraction
of the swimming-bcdls.
Microsco})ic examination shows that the stem consists, like
that of Bougainvillea, of ectoderm, mesogloca, and endo-
derm, but without a cuticle. The hydranth has a similar
structure to that of Bougainvillea, only differing in shape
and in the absence of tentacles round the mouth : the grap-
pling lines are formed on the polype-type : the medusoids are
merely simplified medusae : the swimming-bells are i)ractic-
ally medusae in which the manubrium is absent : and the
bracts are shown by comparison with allied forms to be
greatly modified medu.sa-like structures.
Diphyes is in fact a free-swimming hydroid colony which,
instead of being dimorphic like Bougainvillea, is polymorphic.
In addition to nutritive zooids or hydranths, it possesses
locomotive zooids or swimming-bells, protective zooids or
bracts, and tentacular zooids or grappling-lines. Morpho-
logical and physiological differentiation are thus carried
much further than in such a form as Bougainvillea.
Porpita is another free-swimming Hydroid, presenting at
first sight no resemblance whatever to Diphyes. It has much
the apjxiarance of a flattened medusa (Fig. 59), consisting
of a circular di.sc, slightly convex above and concave below,
bearing round its edge a number of close-set tentacles, and
on its under side a central tubular organ (hy) with a ter-
250
IIYDROID POLYPES
LESS.
Fig. 58. — Diphyes campanniata.
A, the entire colony, natural size, showing stem («) bearing groups of
zooids {f) and two swimming bells (w, w), the apertures of which are
marked 0,
B, one of the groups of zooids marked e in A, showing common stem,
(tf), hydranth («), mediisoid (^), bract (/), and branched tentacle or
grappling line (1*). (From Gegenbaur.)
Fig. 59. — A, Porpiia pacifica (nat, size), from beneath, showing disc-
like stem surrounded by tentacles (/■), a single functional hydranth {hy)^
and numerous moulhless hydranths {hy').
B, vertical section of P. mediterranean showing the relative positions
of the functional {hy) and mouthless {hy') hydranths, the tentacles,
and the chambered shell {sh), (a after Duperrey ; B from Huxley, after
Kolliker.)
The discoid body is supported by a sort of slieli having the
consistency of cartilage and divided into chambers which
contain air (b, sh).
Accurate examination shows that the manubrium-like
252 HYDROID POLYPES less, xxn
body (hv) on the under surface is a hydranth, that the short,
hollow, tentacle like bodies (///) surrounding it are mouthless
hydranths, and that the disc represents the common stem of
Diphyes or l^ougainvillea. So that Porpila is not what it
appears at first sight, a single individual, like a Medusa or a
Hydra, but ii colony in whi('h the constituent zooids have
become so modified in accordance with an extreme division
of physiological labour, that the entire colony has the char-
acter of a single physiological individual.
It was pointed out in the previous lesson (p. 230) that
Hydra, while morphologically the eciuivalent of an indefinite
number of unicellular organisms, was yet physiologically a
single individual, its constituent cells being so differentiated
and combined as to form one whole. A further stage in this
same process of individuation is seen in Porpita, in which not
cells but zooids, each the morphological equivalent of an
entire Hydra, are combined and differentiated so as to form
a colony which, from the physiological point of view, has
the characters of a single individual.
LESSON XXIII
SPKRMATOGENKSIS AND f)0(;i«.NKSIS. TDK MATURATION AND
imprk(;nation of thk ovum, the connection he-
TWP:EN UNICEI.LUI.AR and DIPLOULASTIC ANIMALS
In the preceding lessons it has more than once been staled
that sperms arise from ordinary undifferentiated cells^ in tlie
spermary, and that ova arc produced by the enlargement
of similar cells in the ovary. Fertilisation has also been de-
scribed as the conjugation or fusion of ovum and sperm. We
have now to consider in greater detail what is known as to
the [irecise mode of development of sperms {spermaiosenesis)
and of ova {oozcnesis \ as well as the exact steps of the pro-
cess by which an oosperm or unicellular embryo is formed
by the union of the two sexual elements. 'J'he following
description applies to animals : recent researches show that
essentially similar processes take place in plants.
Both ovary and spermary are at first composed of cells of
the ordinary kind, the primitive sex-cel ls^ and it is only by
the further development of these that the sex of the gonad
is determined.
In the spermary the sex-cells (Fig.6o, a) undergo repeated
fission, forming what are known as the sperm-mother-cells
(b). These have been found in several instances to be
254 SPERMATOGENESIS AND OOGENESIS less.
distiii^iiislu cl by a peculiar condition of the nucleus. We
saw (p. 65 ) that the number of chromosomes is constant in
Fig. 60. — Spt'rnu\to|;cncsis m the A'fole-Cricket [^Gryllotalpd).
A. Primili\e sv\-cc\\, just ])ic|>.ii jtoiy to division, showing twelve
chronio'iouK's (i the cent losniue.
B. Spcim inollici cell, funncil l>y the division of A, and containing
twentN 'tour clnoinosoines. The ceiitrosomc ha^. divided into two.
('. The spci in-mothor-cell has di\ i<led into two by a reducing division,
each daughler-cell containing twchc chromosomes.
i>. ICach daughter-cell has divided again in the same manner, a group
of fom Speim-tells being ]>rodiiced, each w ith six chromosomes.
E. A single sperrn-ccll about to elongate to form a sperm.
F. Immature sperm 5 the six chromosomes are still visible in the
head.
G. Fully formed sperm.
(After von Rath.)
XXIII
REDUCING DIVISION
255
any given animal, though varying greatly in different species.
In the formation of the sperm-mother>cells from the primitive
sex-cells the number becom e s do ubled : in the case of
the mole-cricket, for instance, shown in Fig. 6r, while the
ordinary cells of the body, including the primitive sex-
cells, contain twelves chromosomes, the sperm-mother-cells
contain twenty-four.
The sperm-mother-cell now divides (c), hut instead of its
chromosomes splitting in the ordinary way (p. 64 and Fig. to)
half of their total number —in the present instance twelve - --
passes into each daughter cell : i n this way two cells are
produced having; the normal number of chromosomes. The
process of division is immediately repeated in the same
peculiar wav (n), the result being that each sperm-mother -
cell gives rise to a ^roup of four cells having half the normal
number of chromosomes — in the present instance six. 'I’lie
four cells thus [)roduced arc the i mmature sperms (k) ; in
the majority of cases the protoplasm of each undergoes a
great elongation, being converted into a long vibratile thread,
the of the sperm (f, g), while the nucleus becomes its
more or less spindle-shaped head and the centrosome takes
the form of a small inte rmediate pie ce at the junction of
head and tail.
Thus the sperm or male gamete is a true cell, specially
modified in most cases for active movement: its head,
representing the nucleus, is directed forwards in progres-
sion, its long tail, formed from the protoplasm, backwards.
The direction of movement is thus the precise opposite of
that of a monad (p. 36) to which a sperm presents a certain
resemblance. This actively motile tailed form is, however,
by no means essential : in many animals the sperms are
non-motile and in some they resemble ordinary cells.
The peculiar variety of mitosis described above, by which
256 SPERMATOGENESIS AND OOGENESIS less.
the number of chromosomes in the sperm-mother-cells is
reduced by one-half, is known as a redudm division.
As already stated, the ova arise from primitive sex-cells,
precisely resembling those which give rise to sperms. 'Fhese
divide and kIvo rise to the e^^-moihcr-cells in which, as in
the sperm-mother-cells, t he number of chromosomes is
doubled , ' i'he egg-mother-cells do not immediately undertio
division but remain passive and increase, often enormously,
in size, by the absorption of nutriment from surrounding
parts : in this way each egg-mother-cell become s an ovum .
Sometimes this nutriment is simply taken in by osmosis,
in other cases the growing ovum actually ingests neigh-
bouring cells after the manner of an Amueba. 'Thus in the
developing egg the processes of constructive are vastly
in excess of those of destructive metabolism.
We saw in the second lesson (p. 33) that the products of
destructive metabolism might take the form either of waste
products which are got rid of, or of plastic products which
are stored up as an integral part of the organism. In the
developing egg, in addition to increase in the bulk of the
protoplasm itself, a formation of plastic products usually
goes on to an immense extent. In plants the stored-up
materials may take the form of starch, as in Nitella (p. 214),
of oil, or of proteid substance : in animals it consists of
rounded or angular grains of proteid material, known as
volk -granu/es . These being deposited, like plums in a
pudding, in the protoplasm, have the effect of rendering the
fully-formed egg opaque, so that its structure can often be
made out only in sections. When the quantity of yolk is
very great the ovum may attain a comparatively enormous
size, as for instance in birds, in which, as already mentioned
(p. 69), the “ yolk ” is simply an immense egg-cell.
When fully formed, the typical animal ovum (Fig. 61)
XXIII
STRUCTURE OF THE OVUM
257
consists of a more or less jj;lobular mass of protoj)lasm,
generally exhibiting a reticular structure and enclosing a
larger or smaller quantity of yolk-granules. Surrounding
t he cell body is usually a cell-wall or cuticle , o ften of con-
siderable thickness and known as the vitelline membrane :
fre(iucntly it is perforated at one pole by an aperture, the
viicropyle (fig. 62, microp). 'I'he nucleus is large and has
Fk;. 61. -Oviun of a Sca-urchin [Toxopneustes hvuius), showing the
radmlly-^triatc'd ccll-wall (vitelline membrane), the protoplasm contain-
ing yolk granules (vitellus), the large nucleus (germinal vesicle) with its
network of chromatin, and a large nucleolus (germinal spot). (From
Balfour after Ilcrtwig.)
the usual constituents (p. 63) — nuclear membrane, nuclear
sap, and chromatin. As a rule there is a very definite nucle-
olus, which is often known as the terminal spot s the entire
nucleus being called the yermmal vesicle .
Such a fully-formed ovum is, however, incapable of being
fertilized or of developing into an embryo : before it is ripe
for conjugation with a .sperm or able to undergo the first
stages of segmentation it has to go through a process known
as the maturation of the
s
B
Fio. 62. —The Maturation and Impregnation of the Animal Ovum.
A, the ovum, surrounded by the vitelline membrane (hit'm), in the
act of forming the first polar cell (Jv /) ; 9 centrosome.
n, both polar cells {/^{}/) are formed, the female pronucleiis ( 9 pron)
lies near the centre of the ovum, and one of several sperms is shown
making its way into the ovum at the micropyle {mkrop).
LESS. XXIIl
POLAR CELLS
259
c, the head of the sperm has l>ecome the male pronnclcus ( <5
its intci mediate piece the male centrosonie ( icnt ) ; other structures as
before.
i>, the male and female pronuclei are in the act of conjugation.
E, conjugation is comjdete and the .segmentation nucleus nml)
formed. (From Parker and IlaswelPs
Maturation con.si.sts essentially in a twice-repeated proc ess
qf^ccU-di vision. 'J'hc nurk-us (Fig. 62, A,) lo.scs its nicm-
brane, travels to the surface of the egg, and takes on the
form of an ordinary nuclear spindle. Next the protoplasm
grows out into a small projection or bud, into which one end
of the spindle projects. I'hc usual ])rocess of nuclear
division then takes place (Fig. 10, p. 64), one of the
daughter nuclei remaining in the bud {pol)^ the other in
the ovum itself. Nuclear division is followed as usual by
division of the protoplasm, and the bud becomes separated
as a small cell distinguished as the first polar cell.
26 o
SPERMATOGENESIS AND OOGENESIS
LESS.
The formation of Ijoth polar cells takes place by a
r educing division^ so that, while the immature ovum con-
tains double the number of chromosomes found in the
ordinary cells of the species, the mature ovum, like the
sperm, contains only one-half the normal number.
In some animals the first polar body has been found to
divide after separating from the egg. In such cases the egg-
mother-cell or immature ovum gives rise to a group of four
cells -the mature ovum and three polar-cells ; just as the
sperm-mother-cell gives rise to a group of four cells, all of
which, however, become sperms.
After maturation has taken place , the; ovum is ready to be
f(?rtilixed by the conjugation with it of a single sperm . As
we have found repeatedly, sperms are produced in vastly
greater numbers than ova, and it often happens that a single
egg is seen (juite surrounded with sperms, all apparently
about to conjugate with it. It has however been fo ju^ates:
be a general rule that only one of these actually co. it fulfill-
the others, like most drones in a hive, perish v ’thoi
ing the one function they are fitted to perform. right
T'he successful sperm (ii) takes up a position at
angles to the surface of the egg, and gradually pa.ocs
through the micropyle {microp) or works its way through
the vitelline membrane until its head lies within the egg-
protoplasm. The tail is then cast off and the head, ac -
companied by the intermediate piece or centrosome, pene -
trating deeper into the protoplasm , takes on the form of a
rounded nucleus-likc body, the male projiitckus (c, d pron).
The two pronuclei approach one another (d) and finally
unite to form what is called the sesmentation nucleus (e, seg,
Ht4ci\ the single nucleus of what is not now the ovum but
th e msi>crm — the impregnated egg or unicellular embryo.
The fertilizing process is thus seen to consist of the union
xxin UNICELLULAR AND MULTICELLULAR ANIMALS 261
of two nuclear bodies, one contributed by the male gamete
or sperm, the other by the female gamete or ovum. It
follows from this that the essential nuclear matter or chro-
matin of the oosperm is derived in equal proportions from
each of the two parents.
Moreover, a5 both male and female proniiclei contain only
half the number of chromosomes found in the ordinary cells
of the species, the union of the proniiclei results in the
restoration of the normal number to the oosperm.
In some cases the astrospheres of the sperm and
ovum as well as their nuclei appear to unite with one
another, but more usually the egg-centrosome degenerates
and disappears, the centrosome of the oosperm— and conse-
quently of all the cells of the fully-formed animal— being
derived from the centrosome of the sperm, i.e. from the
male parent.
Fertilization being thus effected, the process of segmenta-
tion or division of the oosperm takes place as described in
the preceding lesson (p. 246).
In concluding the present lesson, we shall consider briefly
a ])6int which has probably already struck the reader.
Among the plant-forms which have come under our ncjtice
there has been a very complete series of gradations from
the simple cell, through the non-cellular filament, linear
aggregate, and superficial aggregate, to the solid aggregate,
whilst among the animals already discussed there has so
far been no attempt to fill up the very considerable gap
between unicellular and multicellular forms. In Amoeba,
Vorticella, &c., the entire animal is a single cell, while
our next animal type. Hydra, is not only a solid aggregate
but has its cells arranged in two definite layers enclosing
a digestive cavity. Moreover, in unicellular organisms repro
262 SJ'KKMATOGKNESIS AND OOGENESIS less, xxiii
(luction is effected either asexually by the fission of the en-
tire individual, or in the case of sexual reproduction, follows
after two entire individuals have undc^rgone conjugation. In
nuillicellular forms, on the other hand, single cells are set
a[)art for sexual reproduction.
When we say that no attempt has been made to fill u{)
this gap, we mean as far as adult forms are concerned. If
the reader will turn to the ac count, in the previous lesson,
of the development of hydroid polypes (p. 246), he will see
that the facts there described do as a matter of fact help
us to see a possible connection between unicellular
animals and multicellular two-layered forms with mouth
and digestive cavity, d'he oosperm of the hydroid (Fig.
57, a) has the essential character of an Amceba, the
polyplast (t:) is practically a colony of Anuebaj, and the
planula (n) a similar colony in which the zooids (cells)
are dimorphic, being arranged in two layers, with a central
('avity which finally communicates with the exterior by a
mouth.
Jt is an interesting circumstance that these embryonic
stages are to some extent paralleled by certain adult
organisms, two of the more accessible and well-known of
which wiU^^jiow be described.
Pandorina (Fig. 63, a) is a colony consisting of sixteen
unicellular zooids closely packed in a gelatinous case of a
globular form. Each zooid resembles in general characters
a motile Ha^matoccus or Eiiglena, having an ovoid cell-body
coloured green by chlorophyll, a red pigment-spot, and
two flagella, which protrude through the gelatinous wall of
the colony, and by their action impart to it a rotatory
movement.
In asexual reproduction each of the sixteen zooids divides
and re-divides, forming at last a group of sixteen cells. Li
Fig. 63 . — Pmidorina monm.
A. The entire colony, consisting of sixteen flagellate zooids, enclosed
in a gelatinous envelope.
B. Asexual reproduction ; each zooid has divided intCTsixtcen, foftning
as many daughter families, still enclosed within the original gelatinous
envelope,
C. Sexual reproduction ; zooids are being set free from the colony,
forming gametes.
D. Conjugation of two gametes.
E. The same after complete fusion.
F. The immature zygote.
G. The fully-formed zygote.
H. Protoplasm of zygote escaping from cell -wall.
I. The same after acquisition of flagella.
K. The same undergoing division and forming a young colony.
(From Goebel.)
264 SPERMATOGENESIS AND OOGENESIS less, xxiii
this way sixteen daughter colonies are produced within the
gelatinous envelope of the original mother colony (u). By
th(j solution of the envelope the daughter colonies are set
free, and each begins an independent existence.
In sexual reproduction the zooids are set free singly from
the colony (c). 'Fhey swim aliout actively, approach one
another in jiairs, and conjugate (d), becoming completely
fused together (k) to form a zygote (k). This increases in
size and develops a thick cell wall ((i). After a period of
rest, the protoplasm escapes from the cell wall (h), puts out
a pair of flagella (i), and swims about. Finally it settles
down, divides and re-divides, and so gives rise to a new
colony (k).
It is obvious that Pandorina resembles the polyplast
stage of an embryo : moreover it is produced by the re-
peated fission of a zygote, just as the polyplast is formed
by the repeated fissitm of an oosperm.
''Phe beautiful Vohwx (Figs 64 and 65), one of the favourite
studies of niicroscopists, is a colony of Ha?matococcus-like
zooids arranged in the form of a hollow sphere containing a
transparent mucilage. Each cell (c) has a nucleus, a con-
tractile vacuole, a large green chromatophore, a small red
pigment-spot like that of Euglena (p 47) and two flagella.
The cells are surrounded by thick mucilaginous cell-walls
which do not give the reaction of cellulose, but are probably
formed of an allied carbohydrate. By the combined move-
ment of all the flagella a rotating movement is given to the
entire colony.
Asexual reproduction takes place by means of certain
zooids distinguished from the rest by the absence of flagella .
and called partheno^onidia (Fig. 64. a, d). Each partheno-
gonidium undergoes a process very like the segmentation of
the hydroid egg (p. 246), dividing into 2, 4, 8, 16, <&c. cells
fl
BlPii
‘A' . !l /I (I \> ' '
D' D®
flF) w !^IRi fefey
A
c ^
Fig. 64. — Volvo X global or.
A, the entire colony, surface view, showing (he biflagellate zooi<l.s and
several daughter-colonies swiniming freely in the interior ; the latter are
produced by the repeated fission of non-flagcllatc reproductive zooids
or parthenogonidia {a)
B, the same during sexual maturity, showing spermaries from the
surface in profile {spy)^ and after complete formation of sperms
{spy ') : and ovaries from the surface (ovy, ovy"., ory") and in jirofilc
c, four zooids in optical section, showing cell -wall, nucleus, contractile
vacuole, with adjacent pigment-spot, and flagella (j?).
id-D^, stages in the formation of a colony by the repeated binary
fission of an asexual rejuoductive zooid.
E, a ripe spermary,
F, a single sperm, showing pigment-spot (pg) and flagella (Jl).
Gf an ovary containing a single ovum surrounded by several sperms.
H, oosperm enclosed in its spinose cell-wall.
(a from Geddes and Thomson, after Kirchner ; B-H after Cohn.)
266
SPERMATOGENESIS AND OOGENESIS
tESS.
(a, Uy and i/'), and so forming a daughter colony which
becomes detac hed and swims freely in the interior of the
|)arent colony (a), by the rii[)tiire of which it is finally
liberated. In sexual re[)rodueti()n certain cells enlarge and
take on the (‘hara('ters of o\aiies (t?, ovy\ ov/'y 07y"y
and Fig. 66, o) the protoplasm of each forming a single
Fig. 65.
P.irt of a Volvox-colony sliowing the structure in greater detail than
in Fig. 64: Sy spenuaiics ; e, ovaries. (From J.ang. )
ovum : the protoplasm of others divides repeatedly and
forms aggregatioips of sperms (h, spyy spy y spy" y and Fig.
65, s), by the conjugation of a sperm (f) with an ovum (g)
an oosperm (h) is produced, and from this by continued
division a new colony arises.
Volvox is clearly comparable to a hollow polyplast, and
further resembles the higher or multicellular animals in that
certain of its cells are differentiated to form true sexual-
products.
It is necessary, in conclusion, to remind the reader that
XXIII UNICELLULAR AND MULTICELLULAR ANIMALS 267
the Mycetozoa and Opalina may be said to take an inter-
mediate place between the strictly unicellular and the multi-
cellularaniinals in much thesameway as Mucorand Vaucheria
connect unicellular and niultitellular plants. 'Ehe plas-
modium of the Mycetozoa is formed, in the first instance
(p. 54), by the fusion of anKcbuke: hence it is a many-celled
structiirti, the ('onstiluent cells of whii'h have lost their
boundaries and are indicated only by their nuclei. Sub-
sequently the nuclei multiply by division, and, although
the process does not affect the protoplasm, it is allowable to
say that the number of virtual cells of whii'h the jilasmodium
is composed is thereby increased. 'I'he Mycetozoon, in its
plasmodial stage, is, in fact, a non-cellular organism, like
Mucor or Vaucheria. l>ut if this way of looking at the
Mycetozoa is correct, it follows that Opalina is to be con-
sidered rather as a multiimcleate but non-cellular than as a
unicellular animal.
LESSON XXIV
rOLYClORDlUS
PoT.vGORDius is a minute worm, about 3 or 4 cm. in length,
found in tlie European seas, where it lives in sand at a
depth of a few hithoms. It lias much the appearance of a
tangle of pink thread with one end produced into two delicate
processes (Fig. 66, a). These, which are the tentacles^ mark
the anterior end of the animal — the ojiposite extremity,
which in some species also bears a pair of slender processes,
is the posterior end. As the creature creeps along, one side
is kept constantly upwards and is distinguished as the dorsal
aspect ; the lower surface is called ventral.
'The anterior end is narrower than the rest of the body,
and is marked off behind by a groove (b and c) ; this
division is called the prostomium {Pr, st) and bears the
tentacles (/) already mentioned in front and above, and on
each side a small oval depression (c. /) lined with cilia.
Immediately following the prostomium is a region clearly
marked off in front, but ill-defined posteriorly, and known as
the peristomium (Per, sP) ; on its ventral surface is a trans-
verse triangular aperture the mouth (Alf/i), The rest of
the body is more or less distinctly marked by annular
grooves (d and e, ^r) into body-segments or metameres
Fig. 66. — Polygo7'diiis ueapoliiamts.
A, the living animal, dorsal aspect, about five times natural size.
B, anterior end of the worm from the right side, more highly magni-
fied, showing the prostomium (/V. st)^ peristomium {Per. x/), tentacles
(/), with seta? (x) and ciliated pit (c. p).
c, ventral aspect of the same : letters as before except mouth.
D, portion of body showing metamercs (Mtmr) separated by grooves
(^)-
E, posterior extremity from the ventral aspect, showing the last three
metameres (Mtmr) sejiaratcd by distinct grooves {gr)y the anal seg-
ment (All, seg) bearing the anus (Aii)y and a circlet of papillx (/).
(After Fraipont.)
270
POLYGORDIUS
LESS XXIV.
{Mtmr\ the number of which varies considerably. Poly-
gordius is thus the first instance we have met with of a trans-
versely segmented animal. The last or anal segment
(e, An, seg) differs from the others by its swollen fprm and
by bearing a circlet of little prominences or papillae {p ) ; it
is separated from the preceding segment by a deep groove
and bears at its posterior end a small circular aperture, the
anus {An).
Polygordius may therefore be described as consisting of a
number of more or less distinct segments which follow one
another in longitudinal series ; three of these, the prostomium^
which lies altogether in front of the mouth, the peristomium^
which contains the mouth, and the a 7 ial segment^ which
contains the anus, are constant and arc distinguished by
special characters ; while between the peristomium and the
anal segment are intercalated a variable number of metameres
which resemble one another in all essential respects.
Polygordius feeds in much the same way as an earth-
worm : it takes in sand, together with the various nutrient
matters contained in it, such as infusoria, diatoms, &c., by
the mouth, and after retaining it for a longer or shorter time
in the body, expels it by the anus. It is obvious, therefore,
that there must be some kind of digestive cavity into which
the food passes by the mouth, and from which effete matters
are expelled through the anus. Sections (Fig. 67) show
that this cavity is not a mere space excavated in the interior
of the body, but a definite tube, the enteric canal (a, b),
which passes in a straight line from mouth to anus, and is
separated in its whole extent from the walls of the body
(a, B. IV) by Si wide space, the Mjy cavity or ccelome (Cal).
So that the general structure of Polygordius might be imi-
tated by taking a wide tube, stopping the ends of it with
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the surface and in transverse section.
The mouth leads into the enteric canal, which is somewhat dilated in each segment, and ends posteriorly in the
anus {An) ; it siu.w n partly entire, partly in section : PJh pharynx ; Oes, oesophagus ; Int, intestine ; Ret, rectum.
^ The diagrammatic figures in this and the follovi ing illustrations to Polygordius are founded upon F raipont s figures,
although not actually copied from them.
272
rOLYGOKDlUS
LESS.
Between the enteric canal and the body-wall is the coelome (Gi/)*
divided into right and left portions by the dorsal (Z>. Mes) and ventral
( V. Mes) mesenteries, and into segmental compartments by the septa
laying in the mesenteries are the dorsal (/A V) and ventral (K V)
blood-vessels, connected by commissural vessels {Com, C) running in
the septa ; from the latter go off recurrent vessels (A', f '),
Nephridia {Nphm) arc shown in the second and third metamercs,
each consisting of a horizon tal portion which perforates a septum and
opens in the preceding segment by a nephrostome {Nph. st) and of a
vertical poition which perforates the body-wall and opens externally by
a nephridioporc {Nph. /),
I'lie brain (Ar) lies in the prostomium and is connected with the
vent ral- nerve cord ( Nv, Cd,) by a pair of oesophageal connectives
{Uis, Com).
M, diagrammatic longitudinal section showing the cell-layers.
The cuticle is represented by a black line, the ectoderm is dotted,
the endoderm radially striated, the muscle plates evenly shaded, the
Cfelomic epithelium repicsented by a beaded line, and the nervous
system finely dotted.
The body-wall is composed of cuticle {Cu), deric epithelium (Der.
Epthm^y muscle-plates (JA /Y), and parietal layer of ccelomic epithe-
lium {Cal. Kpthm).
The enteric canal is foimed of enteric epithelium {Ent. Ei>thm)
covered liy the visceral layer of cieloniic epithelium {Cal, Epthfn') ; in
the neighbourhood of the mouth (JAM) and anus (.<*/;/) the enteric epithe-
lium is ectotlennal ; elsewhere it is endodermal ; /Vt, pharynx ; 0 £S,
oesophagus j y»l, intestine ; A*</, rectum.
The se])ta {Stpl) are formed of muscle covered on both sides by ctelomic
cpilheliiim.
Four nephridia {Np/im) with nephrostome {Npli, si) and nephridioporc
{Nph. p) are shown.
The brain {Er) and ventral nerve cord ( V. Nv. Cd) arc seen to be in
contact with the ectoderm : from the brain a nerve {nv) passes to the
tentacle.
c, diagrammatic transverse section showing the cell-layers as in B,
viz. the cuticle (C//), deric epithelium {Der. Plpthm)^ muscle-plates
{M. 7 /}, ami parietal layer of ccelomic epithelium {Cal. Epihm\ torm-
ing the body-w.all ; and the enteric epithelium {Ent. Epthm) and
visceral layer of ccvlomic epithelium {Cad. Epthm'), forming the enteric
canal.
Thcdoisal {D.Mt’i) and vcntial {V. J/es) mesenteries are seen to be
formed of a double layer of ccelomic epithelium, and to enclose respec-
tively the dorsal (/A F’) and ventral ( K A ) blood-vessels.
A nephridium {Nphm) is shown on each side with nephrostome {Nph,
si) and nephritliopore {Nph.p).
The connection of the ventral nerve-cord with the ectoderm (deric
epithelium) is well shown.
Fig. 70, A (p. 291), should be compared with this figure, as it
is an accurate representation of the parts shown here diagram-
matically.
XXIV
BODY-WALL
273
corks, boring a hole in each cork, and then inserting through
the holes a narrow tube of the same length as the wide one.
The outer tube would represent the body-wall, the inner the
enteric canal, and the cylindrical space between the two the
ccelome. The inner tube would communicate with the ex-
terior by each of its ends, representing respectively mouth
and anus ; the space between the two tubes, on the other
hand, would have no communication with the outside.
Polygordius is the first example we have studied of a
ccelomate animal : one in which there is a definite body*
cavity separating from one another the body-wall and the
enteric canal, and in which therefore a transverse section of
the body has the general character of two concentric circles
(Fig. 67, c).
It will be remembered that a transverse section of Hydra
has the character of two concentric circles, formed re-
spectively of ectoderm and endoderm (Fig. 56, a', p. 241),
the two layers being, however, in contact or separated only
by the thin mesoglcea. At first sight then, it seems as if we
might compare Polygordius to a Hydra in which the ecto-
derm and endoderm instead of being in contact were
separated by a wide interval ; we should then compare the
body-wall of Polygordius with the ectoderm of Hydra and
its enteric canal with the endoderm. But this comparison
would only express part of the truth.
A thin transverse section shows the body-wall of Poly-
gordius to consist of four distinct layers. Outside is a thin
transparent cuticle (Fig. 67, c, and Fig. 70, a, cu ) showing
no structure beyond a delicate striation. Next comes a
layer of epithelium {Der, Epthm\ showing no cell-boundaries
and thus having the character of a sheet of protoplasm with
regularly disposed nuclei : this is the deric epithelium or epi-
dermis. Within it is a rather thick layer of muscle-plates
T
274
rOLYGORDIUS
LESS.
(J/, Pl\ having the form of long flat spindles (Fig. 69, p.
284, M, PL) exhibiting a delicate longitudinal striation and
covered on their free services with a fine network of proto-
plasm containing scattered nuclei. Each plate is arranged
longitudinally, extending through several segments, and with
its short axis perpendicular to the surface of the body (Fig.
70, M. PL), It is by the contraction of the muscle-plates
that the movements of the body, which resemble those of
an earthworm, are produced. Finally, within the muscular
layer and lining the ccelome is a very thin layer of cells, the
Civ/oMic epithelium {Qel. Epthm).
A transverse section of the enteric canal shows only tw'o
layers. The inner consists of elongated cells {Ent, Epthm)
fringed on their inner or free surfaces with cilia : these con-
stitute the enteric epithelium. Outside these is a very thin
layer of flattened cells (CVi 7 . Epthm') bounding the coelome,
and hence called, like the innermost layer of the body-wall,
ctelomic epithelium. We have, therefore, to distinguish
two layers of ccelomic epithelium, an outer or parietal layer
(CoeL Epthm,) which lines the body-wall, and an inner or vis-
ceral layer {Ccel, Epthm') which invests the enteric canal
We are now in a better position to compare the transverse
sections of Hydra and of Polygordius (Fig. 55, a', and Fig.
67, c). The deric epithelium of Polygordius being the
outermost cell-layer is to be compared with the ectoderm of
Hydra, and its cuticle with the layer of the same name
which, though absent in Hydra, is present in the stem of
hydroid polypes such as Bougainvillea (p. 238). 'Fhe enteric
epithelium of Polygordius, bounding as it does the digestive
cavity, is clearly comparable with the endoderm of Hydra.
So that we have the layer of muscle-plates and the two layers
of ccelomic epithelium not represented in Hydra, in which
their position is occupied merely by the mesogloea.
XXIV DIPLOBLASTIC AND TRIPLOBLASTIC FORIVtS 275
But it will be remembered that in polypes there is some-
times found a layer of separate muscle-fibres between the
ectoderm and the mesogloia, and it was pointed out (p. 236)
that such fibres rejiresented a rudimentary intermediate (dl-
layer or mesoderm. Wc may therefore consiiler the must ular
layer and the cielomic epithelium of l\)lygordius as meso-
derm, and we may say that in this animal the mesoderm is
divisible into an outer or somatic laycr^ consisting of the
muscle-plates and the parietal layer of ccelomic epithelium,
and an inner or splanchnic layer^ t'onsisting of the visceral
layer of cmlomic epithelium.^
7 'he somatic layer is in contact with the ectoderm or deric
epithelium, and with it forms the body-wall ; the splanchnii'
layer is in contact with the endoderm or enteric epithelium
and with it forms the enteric canal. The ctelome separates
the somatic and splanchnic layers from one another, and is
lined throughout by ccelomic epithelium.
The relation between the diploblastic polype and the
triploblastic worm may therefore be expressed in a tabular
form as follows —
Bydroid
("uticle . .
Ectoderm .
Mesoderm .
(rudimentary)|
Endoderm
Polygordius.
Cuticle.
Deric ejiithelium or epidermis.
Somatic
layer
Splanchnic
layer
Muscle-plates,
("celomic epithelium
(parietal layer).
C< el om i c ef )i t hel i um
(visceral layer).
ICnteric epithelium.
^ In the majority of the higher animals there is a layer of muscle
between the enteric and cadomic epithelia : in such cases the body-wall
and enteric canal consist of the same layers but in reverse order, the
coclomic epithelium being internal in the or % external in the other.
T 2
276
POLYGORDIUS
LESS.
Strictly s[)caking, this com{)arison does not hold good of
the anterior and posterior ends of the worm : at both mouth
and anus the deric passes insensibly into the enteric epithe-
lium, and the study of development shows (p. 296) that the
cells lining both the anterior and posterior ends of the canal
are, as indi('ated in the diagram (Fig. 67, b), ectodermal. For
this reason the terms deric and enteric epithelium are not
mere synonyms of ectoderm and endoderm respectively.
It is im[)ortant that the student should, before reading
further, understand clearly the general composition of a
trii)loblastic animal as typified by J^olygordius, which may
be summarised as follows. It consists of two tubes formed
of epithelial cells, one within and parallel to the other, the
two being continuous at either end of the body where the
inner tube (enteric epithelium) is in free communication
with the exterior ; the outer tube (deric epithelium) is lined
by a layer of muscle-plates within which is a thin layer of
cielomic epithelium, the three together forming the body-
wall ; the inner tube (enteric epithelium) is covered ex-
ternally by a layer of ccelomic epithelium which forms with
it the enteric canal ; lastly, the body-wall and enteric canal
are setjarated by a considerable space, the coelome.
The enteric canal is not, as might be supposed from the
foregoing description, connected with the body-wall only at
the mouth and anus, but is supported in a peculiar and
somewhat complicated way. In the first place there are
thin vertical plates, the dorsal and ventral mesenteries (Fig.
67, A and c, D. Mes^ V, A/es), which extend longitudinally
from the dorsal and ventral surfaces of the canal to the body
wall, dividing the coelome into right and left halves. The
structure of the mesenteries is seen in a transverse section
(Fig. 67, c, and Fig. 70, a) which shows that at the middle
XXIV
ENTERIC CANAL
277
dorsal line the parietal layer of coelomic epithelium becomes
deflected downwards, forming a two-layered membrane, the
dorsal mesentery ; the two layers of this on reaching the
enteric canal diverge and pass one on either side of it, form'
ing the visceral layer of ctelomic epithelium ; uniting again
below the canal, they are continued downwards as the ventral
mesentery, and on reaching the body-wall diverge once more
to join tile parietal layer. Thus the mesenteries an^ simply
formed of a double layer of coelomic epithelium, continuous
on the one hand with the parietal and on the other with the
visceral layer of that membrane.
Beside the mesenteries, the canal is supported by trans-
verse vertical partitions or septa (Fig. 67*, a and ii, Sept) which
extend right across the body-cavity, each being perforated by
the canal. The septa are regularly arranged and correspond
with the external grooves by which the body is divided into
metameres. Thus the transverse or metameric segmen-
tation affects the coilome as well as the body-wall. Each
septum is composed of a sheet of muscle covered on both
sides with ccelomic epithelium (b, Sepi).
Where the .septa come in contact with the enteric canal,
the latter is more or le.ss definitely constricted, so as to pre-
sent a beaded appearance (a and n) ; thus we have segmen-
tation of the canal as well as of the body-wall and ctelome.
The digestive canal, moreover, is not a simple tube of
even calibre throughout, but is divisible into four portions.
The first or pharynx (P/i) is very short, and can be pro-
truded during feeding ; the second, called the gullet or
esophagus {Oes)y is confined to the perLstomium and is distin-
guished by its thick walls and comparatively great diameter ;
the third or intestine (Int) extends from the first metamere
to the last— -/>., from the .segment immediately following
the peristomium to that immediately preceding the anal
278
POLYGORDIUS
LESS.
segment ; it is laterally compressed so as to have an
elongated form in cross section (c, and Fig. 70, a) : the
fourth f)ortion or rectum (Ret) is confined to the anal seg-
ment ; it is somewhat dilated and is not laterally compressed,
'riie epithelium of the. intestine is, as indicated in the
diagram (u), endodermal ; that of the remaining divisions of
tile canal is ectodermal, d’he large majority of the cells in
all jiarts of tiui canal are ciliated.
d'he cells of the enteric canal and especially those of the
gullet are very granular, and like the endoderm cells of the
hy[)ostf)me of Hydra (p. 228) are to be considered as gland-
cells. They doubtless secrete a digestive juice which,
mixing with the various substances taken in by the mouth,
dissolves the proteids and other digestible parts, so as to
allow of their absorption. There is no evidence of intra-
cellular digestion such as occurs in Hydra (p. 229), and it is
very probable that the process is purely extra-cellular or
enteric, the food being di.ssolved and rendered diffusible
entirely in the cavity of the canal. By the movements of
the canal — caused partly by the general movements, of the
body and partly by the contraction of the muscles of the
septa, aided by the action of the cilia — the contents are
gradually forced backwards and the sand and otlier indi-
gestible matters are expelled at the anus.
The civlome is filled with a colourless, transparent
ccelomic fluid in which are suspended minute, irregular,
colourless bodies, as well as oval bodies containing yellow
granules. From the analogy of the higher animals one
would expect these to be leucocytes (p. 56), but their
cellular nature has not been proved.
The function of the ccelomic fluid is probably to distribute
the digested food in the enteric canal to all parts of the
XXIV
BLOOD-VESSELS
279
body. In Hydra, where the lining wall of the digestive
cavity is in direct contact with the simple wall of the body
the products of digestion can pass at once by diffusion from
endoderm to ectoderm, but in the present case a means of
communication 5s wanted between the enteric epithelium
and the comparatively complex and distant body -wall. 'I'he
peptones and other products of digestion diffuse through
the enteric epithelium into the ccelomic fluid, and by the con-
tinual movement of the latter — due to the contractions of
the body-wall — are distributed to all parts. Thus the
external epithelium and the muscles, as well as the nervous
system and reproductive organs, not yet described, are
wholly dependent upon the enteric epithelium for their
supply of nutriment.
^Ve have now to deal with structures which we find for the
first time in Polygordius, namely blood-vessels. Trying in
the thickness of the dorsal mesentery is a delicate tube (Fig.
67, A and c, D.V.) passing along almost the whole length of
the body r this is the dorsal vessel. A similar ventral vessel
(V.V) is contained in the ventral mesentery,^ and the two are
placed in communication with one another in every segment
by a pair of commissural vessels {A^Com.v) which spring right
and left from the dorsal trunk, pass downwards in or close
behind the corresponding septum, following the contour of
body-wall, and finally open into the ventral vessel. Each
commissural vessel, at about the middle of its length, gives
off a recurrent vessel (R.V.) which passes backwards and
^ The statement that the dorsal and ventral vessels He in the thickness
of the mesenteries requires qualification. As a matter of fact, these
vessels are simply spaces formed by the divergence of the two layers of
epithelium composing the mesentery (Fig. 67, c, and Fig. 70, A) ; only
their anterior ends have proj^er walls.
28 o
POLYGORDIUS
LESS.
ends blindly. The anterior parts of the commissural vessels
lie in the peristomium and have an oblique direction, one on
each side of the gullet. The whole of these vessels form a
single, closed vascular system, there being no communication
between them and any of the remaining cavities of the
body.
I1ie vascular system contains a fluid, the bloody which
varies in colour in the different species of Polygordius, being
either colourless, red, green, or yellow. In one species cor-
puscles (? leucocytes) have been found in it.
'Phe function of the blood has not been actually proved
in Polygordius, but is well known in other worms. In the
common earthworm, for instance, the blood is red, the colour
being due to the same pigment, hicmoglohin^ which occurs
in our own blood and in that of other vertebrate animals.
Haemoglobin is a nitrogenous compound, containing, in
addition to carbon, hydrogen, nitrogen, oxygen, and sulphur,
a minute quantity of iron. It can be obtained pure in the
form of crystals w^hich are soluble in water. Its most
striking and i)hysiologically its most important property is
its power of entering into a loose chemical combination with
oxygen. If a solution of haemoglobin is brought into contact
with oxygen it ac(|uires a bright scarlet colour, and the solu-
tion is then found to have a characteristic spectrum distin-
guished by two absorption-bands, one in the yellow, another
in the green. Loss of oxygen changes the colour from scarlet
to purple, and the spectrum then presents a single broad
absorption-band intermediate in position between the two of
the oxygenated solution.
This property is of use in the following way. All parts
of the organism are constantly undergoing destructive meta-
bolism and giving off carbon dioxide : this gas is absorbed
by the blood, and at the same time the haemoglobin gives up
XXIV
EXCRETORY ORGANS
28r
its oxygen to the tissues. On the other hand, whenever the
blood is brought sufficiently near the external air — or water
in the case of an aquatic animal — the opposite process takes
place, oxygen being absorbed and carbon dioxide given off.
Haemoglobin is therefore to be looked upon as a respiratory
or oxygen-carrying pigment ; its function is to provide the
various parts of the body with a constant supply of oxygen,
while the carbon dioxide formed by their oxidation is given
up to the blood. The particular part of the body in which
the carbon dioxide accumulated in the blood is exchanged
for the oxygen of the surrounding medium is called a
respiratory organ ; in Polygordius, as in the earthworm and
many other of the lower animals, there is no specialised
respiratory organ — lung or gill— but the necessary exchange
of gases is performed by the entire surface of the body.
In discussing in a previous lesson the differences between
plants and animals, we found (p. 178) that in the unicellular
organisms previously studied, the presence of an excretory
organ in the form of a contractile vacuole was a characteristic
feature of such undoubted animals as the ciliate Infusoria,
but w'as absent in such undoubted plants as Vaucheria and
Mucor. But the reader will have noticed that Hydra and its
allies have no specialised excretory organ, waste products
being apparently discharged from any part of the surface.
In Polygordius we meet once more with an animal in which
excretory organs are present, although, in correspondence
with the complexity of the animal itself, they are very
different from the simple contractile vacuoles of Paramoe-
cium or Vorticella.
The excretory organs of Polygordius consist of little tubes
called nephridia^ of which each metamere possesses a pair,
one on either side (Fig. 67, a, b, and c, Nphm), Each
282
POLYGORDIUS
LESS.
nephridiurn (Fig. 68) is an extremely delicate tube consisting
of two divisions bent at right angles. The outer division is
placed vertically, lies in the thickness of the body-wall, and
opens externally by a minute aperture, the nephridiopore
(Figs. 67 and 68, Nph. p). I’he inner division is horizontal
and lies in the roilomie epithelium , passing forward it pierces
the septum which bounds the segment in front (Fig. 67,
A and li), and then dilates into a funnel-shaped extremity or
jiephrostome {Nph. st\ which places its cavity in free com-
munication with the coelome. 'Phe whole interior of the
tube as well as the inner face of the nephrostome is lined
with cilia which work outwards.
Fig. 68. — A nephridhim of Polygordius, showing the cilia lining the
tul)e, the ciliated funnel or nephrostome {Nph. s/), and the external
aperture or nephridiopore (Aph. h), (After Fraipont. )
A nephridiurn may therefore be defined as a ciliated tube,
lying in the thickness of the body-wall and opening at one
end into the coelome and at the other on the exterior of
the body.
In the higher worms, such as the earthworm, the nephridia
are lined in part by gland-cells, and are abundantly supplied
with blood-vessels. Water and nitrogenous waste from all
parts of the body pass by diffusion into the blood and are
conveyed to the nephridia, the gland-cells of which withdraw
the waste products and pass them into the cavities of the
tubes, whence they are finally discharged into the surround-
ing medium. In all probability some such process as this
takes place in Polygordius-
XXIV
NERVOUS SYSTEM
283
In discussing the hydroid polypes we found that one of
the most important points of difference between the loco-
motive medusa and the fixed hydranth was the presence in
the former of a well-developed nervous system (p. 243) con-
sisting of an arrangement of peculiarly modified cells, to
which the function of automatism was assigned. It is
natural to ex})ect in such an active and otherwise highly-
organised animal as l^olygordius a nervous system of a
considerably higher degree of complexity than that of a
medusa.
T'he central nervous system consists of two parts, the
brain and the ventral nerve-cord, 'The brain (Fig. 67, a and
B, Bri) is a rounded mass occupying the whole interior of
the prostomium and divided by a transverse groove into two
lobes, the anterior of which is again marked by a longitu-
dinal groove. I'he ventral nerve-cord ( V, Nv. Cd.) is a
longitudinal band extending along the whole middle ventral
line of the body from the peri.stomium to the anal segment.
The posterior lobe of the brain is connected with the anterior
end of the ventral nerve-cord by a pair of nervous bands,
the oesophageal connectives (CEs. Con.) which pass respectively
right and left of the gullet.
It is to be noted that one division of the central nervous
system — the brain — lies altogether above and in front of the
enteric canal, the other division — the ventral nerve-cord —
altogether beneath it, and that, in virtue of the union of the
two divisions by the oesophageal connectives, the enteric
canal perforates the nervous system.
It is also important to notice that the nervous system is
throughout in direct contact with the epidermis or ectoderm,
the ventral cord appearing in sections (Fig. 67, c, and Fig.
70, a) as a mere thickening of the latter.
Both brain and cord are composed of delicate nerve-fibres
284
POLYGORDIUS
LESS.
(Fig. 69, Nv. F,) interspersed with nerve-cells {Nv. C), In
the cord the fibres are arranged longitudinally, and the
nerve-cells are ventral in position, forming a layer in imme-
iir.P7
Fio. 69. — Diagram illustrating the relations of the nervous system of
Polygordius.
The deric epithelium {Der. Epthm) is either in direct contact with the
central nervous system (lower part of figure), or is connected by aflferenl
nerves {af. nv) with the inter-muscular plexus {int. muse, pix ) : the
latter is connected with the muscle-plates {M. PI) by efferent nerves
{Ef, nv).
The central nervous system consists of nerve-fibres {Nv. F) and
nerve-cells {Nv. C) : other nerve cells {Nv. O') occur at intervals in
the inter-muscular plexus.
The muscle-plates { 3 /. /V), one of which is entire, while only the
middle part of the other is shown, are invested by a delicate protoplasmic
network, containing nuclei {nu), to which the efferent nerves can be
traced. (The details copied from Fraipont.)
diate contact with the deric epithelium. In the posterior
lobe of the brain the nerve-cells are superficial and the
central part of the organ is formed of a finely punctate
XXIV NERVOUS SYSTEM 285
substance in which neither cells nor fibres can be made
out.
Ramifying through the entire muscular layer of the body-
wall is a network of delicate nerve-fibres (////. muse, plx.)
with nerve-cells (Nv. O) at intervals, the inter-muscular
plexus. Some of the branches of this plexus are traceable
to nerve-cells in the central nervous system, others {af. nv.)
to epidermic cells, others (Ef. nv.) to the delicate proto-
plasmic layer covering the muscle-plates. The superficial
cells of both brain and cord are also, as has been said, in
direct connection with the overlying epidermis, and from the
anterior end of the brain a bundle of nerve-fibres (Fig 67, ii,
Nv.) is given off on each side to the corresponding tentacle,
constituting the nerve of that organ, to the epidermic cells of
which its fibres are distributed.
We see then that, apart from the direct connection of
nerve-cells with the epidermis, the central nervous system is
connected, through the intermediation of nerve-fibres {a)
with the sensitive cells of the deric epithelium and {b) with
the contractile muscle-plates. And we can thus distinguish
two sets of nerve-fibres, {a) sensory or afferent (af. nv.)
which connect the central nervous system with the epidermis,
and (b) motor or efferent (Ef. nv.) which connect it with the
muscles.
Comparing the nervous system of Polygordius with that
of a medusa (p. 243) there are two chief points to be noticed.
Firstly, the concentration of the central nervous system in
the higher type, and the special concentration at the anterior
end of the body to form a brain. Secondly, the important
fact that the inter-muscular plexus is not, like the peripheral
nervous system of a medusa which it resembles, situated
immediately beneath the epidermis (ectoderm) but lies in the
muscular layer, or, in other words, has sunk into the
mesoderm.
286
POLYGORDIUS
LESS.
It is obvious that direct experiments on the nervous system
would be a very difficult matter in so small an animal as
Polygordius. But numerous experiments on a large number
of other animals, both higher and lower, allow us to infer
with considerable confidence the functions of the various
parts in this particular case.
If a muscle be laid bare or removed from the body in a
living animal it may be made to contract by the application
of various stimuli, such as a smart tap (mechanical stimulus),
a drop of acid or alkali (chemical stimulus), a hot wire (ther-
mal stimulus), or an electric current (electrical stimulus). If
the motor nerve of the muscle is left intact the application
to it of any of these stimuli produces the same effect as its
direct application to the muscle, the stimulus being con-
ducted along the eminently irritable but non-contractile
nerve.
Further, if the motor nerve is left in connection with the
central nervous system, />., with one or more nerve-cells,
direct stimulation of these is followed by a contraction, and
not only so, but stimulation of a sensory nerve connected
with such cells produces a similar result. And 1\nally,
stimulation of an ectoderm cell connected, either directly
or through the intermediation of a sensory nerve, with the
nerve-cells, is also followed by muscular contraction. An
action of this kind, in which a stimulus applied to the free
sensitive surface of the body is transmitted along a sensory
nerve to a nerve-cell or group of such cells and is then, as it
were, reflected along a motor nerve to a muscle, is called a
reflex action ; the essence of the arrangement is the inter-
position of nerve-cells betw’een sensory or afferent nerves
connected w ith sensory cells, and motor or efferent nerves
connected with muscles.
The diagram (Fig. 69) serves to illustrate this matter.
The muscle-plate ( J/. PL) may be made to contract by a
XXIV ORGANS OF SENSE 287
stimulus applied (a) to itself directly, (fi) to the motor fibre
{Ef, nv\ (c) to the nerve-cells {Nv, C) in the central
nervous system, or to those {Nv, C) in the inter-muscular
plexus, (d) to the sensory fibre {af, nv.), or (e) to the
epidermic cells {Der. Epthm).
In all probability the whole central nervous system of
Polygordius is capable of automatic action. It is a well-
known fact that if the body of an earthworm is cut into
several pieces each performs independent movements ; in
other words, the whole body is not, as in the higher animals,
paralysed by removal of the brain. There can, however, be
little doubt that complete co-ordination, i.e., the regulation
of the various movements to a common end, is lost when
the brain is removed.
The nervous system is thus an all-important means of
communication between the various parts of the organism
and between the organism and the external world. The
outer or sensory surface is by its means brought into
connection with the entire muscular system with such
perfection that the slightest touch applied to one end of the
body may be followed by the almost instantaneous contrac-
tion of muscles at the other.
In some species of Polygordius the prostomium bears a
pair of eye-specks, but in the majority of species the adult
animal is eyeless, and, save for the ciliated pits (Fig. 66,
B, c.p), the function of which is not known, the only definite
organs of sense are the tentacles, which have a tactile
function, their abundant nerve-supply indicating that their
delicacy as organs of touch far surpasses that of the general
surface of the body. They are beset with short, fine pro-
cesses of the cuticle called set(B (Figs. 66 and 67, s), which
probably, like the whiskers of a cat, serve as conductors of
external stimuli to the sensitive epidermic cells.
288
POLYGORDIUS
LESS.
There are two matters of general importance in connec-
tion with the structure of Polygordius to which the student^s
attention must be drawn in concluding the present lesson.
Notice in the first place how in this type, far more than in
any of those previously considered, we have certain definite
parts of the body set apart as organs for the performance of
particular functions. 'Fhere is a mouth few* the reception of
food, an enteric canal for its digestion, and an anus for the
extrusion of faeces : a ca*lomic fluid for the transport of the
products of digestion to the more distant parts of the body :
a system of blood-vessels for the transport of oxygen to and
of carbon dioxide from all parts : an epidermis as organ of
touch and of respiration : nephridia for getting rid of water
and nitrogenous waste : and a definite nervous system for
regulating the movements of the various parts and forming
a means of communication between the organism and the
external world. It is clear that differentiation of structure
and division of physiological labour play a far more obvious
and important part than in any of the organisms hitherto
studied.
Notice in the second place the vastly greater complexity
of microscopic structure than in any of our former types.
The adult organism can no longer be resolved into more or
less obvious cells. In the deric, enteric, and ccelomic
cpithelia we meet with nothing new, but the muscle-plates
are not cells, the nephridia show no cell-structure, neither do
the nerve-fibres nor the punctate substance of the brain,
llie body is thus divisible into tissues or fabrics each clearly
distinguishable from the rest. We have epithelial tissue,
cuticular tissue, muscular tissue, and nervous tissue : and
the blood and ccelomic fluid are to be looked upon as
liquid tissues. One result of this is that, to a far greater
extent than in the foregoing types, we can study the
morphology of Polygordius under two distinct heads:
XXI Y ANATOMY AND IIISTOLOGV 2S9
anatomy^ dealing with the general structure of the |)arts,
and histology^ dealing with their minute or mieroseo[)ie
structure.
One point of importance must be specially referred to in
connection with certain of the tissues It has been pointed
out (p. 273) that the epidermis has rather the character of
a sheet of protoplasm with regularly-arranged nuclei than of
a layer of cells, and that the muscle-plates are covered with
a layer of protoplasm with which the ultimate nerve-fibres
are continuous (p. 274). Thus certain of the tissues of
Polygordius are multinucleate but non-cellular. T'liey are
comparable in minute structure to an Opalina or to the
plasmodium of a Mycotozoon, and must therefore be dis-
tinguished from such definitely cellular tissues as the enteric
epithelium.
U
T.ESSON XXV
I \:) r.YGOR I >i us ( Con finned)
Asioxual reproduction is unknown in Polygordius, and
the organs of sexual reproduction are very simple. The
animal is dicecious, gonads of one sex only being found in
each individual.
In the species which has been most thoroughly investi-
gated {P, neapolitanus) the reproductive products are formed
in each metamere from the fourth to the last. Crossing
these segments oblicpiely are narrow bands of muscle (Fig.
70, A, 0 ,M) and certain of the cells of coelomic epithelium
covering these bands multijdy by fission and form little
heaps of cells {^Spy)^ each of which is to be looked upon as a
gonad. There is thus a pair of gonads to each segment with
the exception of the prostomium, the pcristomium, the first
three metameres, and the anal segment, the reproductive
organs exhibiting the same simple metameric arrangement
as the digestive, excretory, and circulatory organs. It will
be noticed that the primitive sex-cells, arising as they do
from coelom ic epithelium, are mesodermal structures, not
ectodermal as in hydroids (pp. 231 and 245).
In the male the primitive sex-cells divide and sub-divide,
the ultimate products being converted into sperms (Fig. 70,
Fu;. ’]Q.- -Poly}^ordin<^ 7ieapolitamt%.
A, tian^vi T-sc section of a male specimen to show the j)osition of the
immature ^emads {spy} an<l the precise form ami arran^^ement of tlie
vaiious la\ei> rejnesented diaf^i ammatically in Fi|^. 67, c.
The body-wall consists of cuticle (C«), deric epitlieliuin {Per. JCpthm),
muscle-platCN (d/. PI), and parietal layer ol cm, ]f>iiiie ejijihelium {CaL
Rpthm). The ventral nerve cord ( V. Nz>. Cd) i'. ^llo,^n i(i be continu-
ous with the deric epithelium.
The enteric canal consists of ciliated enteric epithelium {Eiit. Jipthfu)
covered by the visceral layer of ccelomic epithelium {C(cl. hpihnt') :
connecting it with the body-wall are the dorsal and ventral mescnierif ^
formed of a double layer of ccelomic epithelium, and containing rcsjjcc-
tively the dorsal {P P) and ventral {P. V) blood-vessels.
Passing obliquely aciuss the coelome are the oblique muscles {O. M)
F 2
292
POr.YGORDIUS
LRSS.
covered with cfdomic epithelium : by differentiation of groups of celK
of the latter the spermaries (S/ty) are formed.
B, a single sperm, showing expande<l head and delicate tail,
c, horizontal section of a sexually matuic female
The body- wall (C/^ Dcr. Epthniy M. PI) has undergone partial
hi.stological degeneration, and is rupture<l in two places to allow of the
escape of the ova {o7>) which still fill the co’loinic spaces enclosed between
the body-wall, the enteric canal Epthm)^ and the septa
(After Fraipont.)
n: see p. 255): in the female they enlarge immensely, and
lake on tlie charac ter of ova (c, ov). Multiplication of the
sexual products takes place to such an extent that the whole
(■(elome becomes ( rammed full of either sperms or ova (c).
In the female the growth of the eggs takes place at the
ex[)ense of all other parts of the body, which underg(^ more
or less complete atrophy : the epidermis for instance, be-
('omes licpiefied and the muscles lose their contractility.
Finally rupture of the body-wall takes place in each segment
(c), and through the slits thus formed the eggs escape. So
that Polygordius, like an annual plant, produces only a
single brood ; death is the inevitable result of sexual
maturity. A\’hether or not the same dehiscence of the body-
wall takes j)lace in the male is not certain : it has been stated
that the sperms make their escape through the nephridia.
Thus while there are no S|)ecialized gofwducis^ or tubes for
carrying off the sexual products, it is possible that the ne-
phridia may, in addition to their ordinary function, sene
the purpose of male gonoducts or spermiducts, F emale gono-
ducts or oviducts are however entirely absent.
The ova and sperms being shed into the surrounding
water, impregnation takes place, and the resulting oosperm
undergoes segmentation or division (see p. 246), a polyplast
being formed. The cells of the polyplast become differen-
tiated, an enteron or digestive cavity is formed, and the
XXV
rriE TROCHOSPHERE
293
eiiibryp is gradually converted into a curious free-swimming
creature shown in Fig. 71, A, and called a trocliosphcre.
The trochosphere, or newly-hatched larva of Polygordius
(Fig. 71, a) is about ] mm. in diameter, and has .something
Fio. 71.— A, larva of roJy^^ordius neapoUtavm in tlic* liochosphcre
stage ; from a living specimen.
B, diagiammatic vertical section of the same : the ectoderm is dotted,
the endoderm radially striated, the mesoderm evenly shaded, and the
nervous system finely dotted.
c, transverse section through the plane ab in B.
The body-wall ctmsists of a single layer of ectoderm cells, which, at
the apex of the prostomium (upper hemisphere) are modified to form the
brain (/>>) and a pair of ocelli {ol).
The enteric canal consists of three ))arts ; the stomodaaim (.SV. (hn)^
opening externally by the mouth and litied by ectoderm ; the
enteion {Ent) lined by endoderm ; and the proctodx'um (/Vr. <////),
opening i)y the anus [An) and lined by ecto<lerra.
Jk'twcen the body-wall an<l the enteric canal is the larval body-cavity
or blastocfele {Bl. cal).
The mesoderm is confined to two narrow bands of cells (b and c,
Msd) in the blastocoele, one on either side of the proctodaeum ; slender
mesodermal bands {Afsd') are also seen in the prostomium in A.
The cilia consist of a prie-oral circlet {Fr. or. ci) above the mouth, a
post-oral circlet (/V. or. d) below the mouth, and an anal circlet {An.
ci) around the anus.
(A after Fraipont.)
the form of a top, consisting of a dome-like upper portion,
the prosto 7 nium, produced into a projecting horizontal rim ;
of an intermediate portion or peristofnium, having the form
of an inverted hemisphere ; and of a lower somewhat conical
294
rOLVGORDIUS
l.ESS.
anal region. Around the projecting rim is a double circlet
of large cilia {Pr. or. ci) by means of which the larva is
propelled through the water.
Beneath the edge of the ciliated rim is a rounded aperture,
the mouth (Mt/i ) ; this leads by a short, nearly straight
gullet {St, dm), into a spacious stomach {Rnt), from the
lower side of which pro(*eeds a short slightly curved intestine
{Prc, dm), opening at the extremity of the conical inferior
region by an anus (An). Between the body-wall and the
enteric canal is a space filled with fluid {P/. coel ), but, as we
shall see, this does not correspond with the body-cavity of the
adult. The body-wall and the enteric canal consist each of
a single layer of epithelial cells, all the tissues included in
the adult under the head of mesoderm (p. 275) being absent
or so poorly developed that they may be neglected for the
present.
Leaving aside all details, it will be seen that the trocho-
sphere of Polygordius is comparable in the general features
of its organization to a medusa (compare Fig. 55, p. 241),
consisting as it does of an outer layer of cells forming the
external covering of the body and of an inner layer lining
the digestive cavity. There are, however, two important
differences : the space between the two layers is occupied by
the mesogkea in the medusa, while in the worm it is a cavity
filled with fluid ; and the digestive cavity of the trochosphere
has two openings instead of one.
But in order to compare more accurately the medusa
with the trochosphere, it is necessary to fill up, by the help
of other types, an important gap in our knowledge of the
development of Polygordius -the passage from the polyplast
to the trochosphere. From what we know of the develop-
ment of other worms, the process, in its general features,
is probably as follows : —
The polyplast is converted, by the accumulation of fluid
XXV
FOK^IATION OF TROCIIOSIMIKRK
295
in its interior, into a hollow sphere, hounded by a single
layer of cells and containing a cavity, the blasioiwle : this
stage of development is ('ailed the blastula. Next, one side
of the blastula becomes tucked in or invaginated so as to
convert the embryo from a single-layered sphere into a
double-layered cup (Fig. 72, a). I'his t)rocess can he
sufficiently well imitated by pushing in one side of
a hollow india-rubber ball. 'I'he resulting embryonic stage
Fici. 72. — Diagram illustrating the origin of the troc)iosj)lierc from
the gastrula. The ectoderm is doited, the endoderm stiiated.
A, gastrula, with enleron {Enl) and gastnila-mouth Mdi), and
with the ectoderm and endoderm separated by the larval body-cavity or
blastoccele {Bi. ari).
M, the gastrula-moiith has closed, the enteron [lint) bcct^ming a shut
sac.
c, two ectodermal pouches, the stomodicum {Sf, dm) and proclexheum
(yVr. dm) have appeared.
D, the stomodaeum (.V/. dm) an«l proctod;eum (/Vr. dm) have opened
into the enteron (Eni), forming a complete enteric canal with mouth
[A/th) and anus {An).
is known as the gastrula : its cavity is the enteron {Ent) and
is bounded by the invaginated cells which now con-
stitute the endoderm, the remaining cells, forming the outer
wall of the gastrula, being the ectoderm. The two layers
are continuous at the aperture of the cup, the gastrula-
mouth or blastopore (Gast. Mth). Ikdween the ectoderm
and endoderm is a space, the greatly diminished hlastcK'jelc.
The resemblance of the gastrula to a simplified Hydra,
devoid of tentacles, will be at once apparent
296
POLYGORDIUS
LESS.'
Before long the mouth of the gastrula closes (i?), the enteron
\Eni) being thus converted into a shut sac. At about the same
time the ectoderm is tucked in or invaginated at two places
(C), and the two little pouches {St. dm^ Pn. dm) thus formed
grow inwards until they meet with the closed enteron and
finally open into it (/>), so that a complete enteric canal is
formed formed, we must not fail to notice, of three distinct
parts : (i) an anterior ectodermal pouch, opening externally
by the mouth, and distinguished as the stomodceum , (2) the
enteron, lined with endoderm ; and (3) a posterior ectoder-
mal pouch, opening externally by the anus, and called the
tyroctodieiim^ ^
In tlie trochosphere (Fig. 71) the gullet is derived from
the stomodieum, the stomach from the enteron, and the
intestine from the proctodieum ; so that only the stomach of
the worm-larva corresponds with the digestive cavity of a
medusa : the gullet and intestine are structures not repre-
sented in the latter form.
Two or three other points in the anatomy of the trocho-
sphere must now be referred to.
At the apex of the dome-shaped prostomium the ecto-
derm is greatly thickened, forming a rounded patch of cells
(Figs. 71 and 73, Br\ the rudiment of the brain. On the
surface of the same region and in close relation with the
brain is a [mir of small patches of black pigment, the
eye-spots or ocelli {Oc).
On either side of the intestine, between its epithelium and
the external ectoderm, is a row of cells forming a band
which partly blocks up the blastoccele (b and c, Msd). These
two bands are the rudiments of the whole of the meso-
dermal tissues of the adult — muscle, coelomic epithelium,
&c. — and are hence called mesodermal bands.
XXV
THE TROCHOSPHERE
297
E'inally on either side of the lower or posterior end of the
stomach is a delicate tube (Fig. 73, a, Nph) opening by a small
aperture on to the exterior, and by a wide funnel-shaped
B, diagrammatic vertical section of the same : the ectoderm is coarsely,
the nervous system finely, dotted, the endoderm radially striated, and
the mesoderm evenly shaded.
c, transverse section through the plane ab in B.
The pre-oral (/V. 07 \ ci)^ post-oral (/V. or, a), and anal {.-In, ci)
cilia, brain ocelli blastocoelc ( 7 //), mouth {Mih), stomo-
dxum {St, dm), proctodoeum {Pre, dm), and anus {An) as in Eig. 71,
the enteron {Ent) has extended some distance into the trunk.
In A, slender mesodermal bands (Msd, bd) in the prostomium, and the
branched head-nephridium {Nph) arc shown.
In B and C the mesoderm {Msd) is seen to have obliterated the blasto-
coele in the trunk-region : the ectoderm has undergone a thickening,
forming the ventral nerve-cord ( V. Nv. Cd).
(a after Fraipont.)
extremity into the blastocoelc : it has all the relations of a
nephridium, and is distinguished as the head~nephdridiutn.
As the larva of Polygordius Is so strikingly different from
the adult, it is obvious that development must, in this, as in
298
POLYGORDIUS
LKbh.
several cases which have come under our notice, be accom-
panied by a metamorphosis.
'Fhe first obvious change is the elongation of the conical
anal region of the trochosphere into a tail-like portion
which may be called the trunk (Fig. 73, a). The
stomach (enteron), which was formerly confined to the pro-
and peri-stomium, has now grown for a considerable
distance into the trunk (b, ent)^ so that the procto-
deum {Prc. dm) occupies only the portion in proximity to
the anus.
Important internal clianges have also taken place. The
deric epithelium or external ectoderm is for the most part
composed, as in the preceding stage, of a single layer of
cells ; but on that aspect of the trunk which lies on the same
side as the mouth - />., to the left in Fig. 73, a and b — this
layer has undergone a notable thickening, being now com-
posed of several layers of cells. This ectodermal thickening
is the rudiment of the ventral nerve-cord ( V, Nv. Cd\ and
the side of the trunk on which it appears is now definitely
marked out as the ventral aspect of the future worm, the
o[)posite aspect — that to the right in the figures — being
dorsal. At a later stage two ectodermal cords — the oesopha-
geal connectives — are formed, connecting the anterior end of
the ventral nerve-cord with the brain. Note that the two
divisions of the central nervous system are originally quite
distinct.
The mesodermal bands, which were small and (juite
separate in the preceding stage (Fig. 71, b and c, Msd),
have now incTeased to such an extent as completely to sur-
round the enteron and obliterate the blastoccele (Fig. 73, b
and c, Msd). At this stage therefore there is no body-
cavity in the trunk, but the space between the deric and
enteric epithelia is occupied by a solid mass of mesoderm.
XXV
METAMORPHOSIS
299
In a word, the larva is at present, as far as the trunk is con-
cerned, triploblastic but ac(elomate.
Development continues, and the larva assumes the form
shown in Fig. 74, a. I'he trunk has undergone a great
increase in length and at the same time has become divided
by a series of annular grooves into segments or metameres,
like those of the adult worm but more distinct (compare
Fig. 66, D, p. 269). By following the growth of the larva
from the preceding to the j)resent stage, it is seen that these
segments are formed from before backwards, />., the seg-
ment next the peristomium is the oldest, and new ones are
continually being added between the last formed and tlui
extremity of the trunk, or what may now be called the anal
segment. By this process the larva has assumed the apj)ear-
ance of a worm with an immense head and a very slender
trunk.
The original larval stomach (enteron) has extended, with
the formation of the metameres, so as to form the greater
portion of the intestine : the proctodaeum (/Vr. dm) is
confined to the anal segment.
Two other obvious changes are the aj)pearance of a pair
of small slender processes (a, /)--the rudiments of the
tentacles — on the apex of the prostomium, and of a circlet
of cilia (/y. an. d) round the posterior end of the trunk.
The internal changes undergone during the assumption of
the present form are very striking. In every fully formed
metamere the mesoderm — solid, it will be remembered,
in the previous stage-- has become divided into two layers,
a somatic layer (h and c, Msd (som) ) in conta('t with the
ectoderm and a splanchnic layer {Msd {spl) ) in contact
with the endoderm. The space between the two layers
{Ccsl) is the permanent body-cavity or coelome, which is
Fu;. 74. — A, larva of Polygordius ncapolitanns in a condition inter-
mediate l)etwcen the trochosphere and the adult worm, the tnink-region
being elongated and divided into metameres.
B, diagrammatic vertical section of the same: the ectoderm is coarsely,
the nervous system finely, dotted, the endoderm radially striated, and
the mesoderm evenly shaded.
c, transverse section along the plane ah in B.
The pre-oral (/V. or. </), post-oral (/V. or. r/), and anal {A)i. ci)
cilia, the blastocade {Bl. rrt’/), stomod;eum {St. doi)^ and proctodieum
{Prc. dm) are as in Fig. 71, A and B : theenteron now extends through-
out the segmented region of the trunk.
A pair of tentacles (/) has appeared on the prostomium near the ocelli
(tf), and a pre-anal circlet of cilia (/V. a)i. ci) is developed.
The mestjderm has divided into somatic {Msd {som) ) and splanchnic
{Msd {spl) ) layers with the coelome {Cad) between : the septa {Sep) are
formed by undivided plates of mesoderm separating the segments of the
cadome from one another.
d'-d^, three stages in the development of the somatic mesoderm. In
I)' it {A/sd (Som) ) consists of a single layer of cells in contact with the
deric epithelium {Ber. Eptkm ) : in D- the cells have begun to split up
in a radial direction : in i)® each has divided into a number of radially
arranged sections of muscle-plates ( J/. Pi) and a single cell of coelomic
epithelium {Cad. Epthm).
(A after Fraipont. )
XXV
METAMORPHOSIS
30t
thus quite a different thing from the larval body -cavity
or hlastoctele, being formed, not as a space between
ectoderm and endoderm, but by the splitting of an
originally solid mesoderm.
The division of the mesoderm does not however extend
quite to the middle dorsal and middle ventral lines : in both
tliese situations a layer of undivided mesoderm is left (c),
and in this way the dorsal and ventral mesenteries are
formed. Spaces in these, apparently the remains of the
blastocHcle, form the dorsal and ventral blood-vessels. More-
over the splitting process takes place independently in each
segment, and a transverse vertical layer of undivided
mesoderm (n, Sep) is left separating each .segment from the
adjacent ones before and behind : in this way the septa
arise.
'The nephridia apj)ear to have a double origin, the super-
ficial portion of each being formed from ectoderm, the
deep portion, including the nephrostome, from the somatic
layer of mesoderm.
In the ventral nerve-cord the cells lying nearest the outer
surface have enlarged and formed nerve-cells, while those on
the dorsal aspect of the cord have elongated longitudinally
and become converted into nerve-fibres. This process has
already begun in the preceding stage.
But the most .striking hi.stological changes are those which
gradually take place in the somatic layer of mesoderm. At
first this layer con.sists of ordinary nucleated cells (i>^, Msd
Sofn\ but before long each cell splits up in a radial
direction (d^) from without inwards from the ectoderm
{Der. Epthni) towards the coelome — finally taking on the
form of a book with four or more slightly separated leaves
directed outwards or towards the surface of the body, and
with its back — the undivided portion of the cell — bounding
302
POLYCORDIUS
[.ESS.
the rcjblonu*. The cells being arranged in longitudinal series,
\V(* havt‘ a number of such books placed end to end in
a row with the corresj)onding leaves in contact —page one
of the first book being followed by page one of the second,
third, fourth, &c., page two by page two, and so on through
one or more segments of the trunk. Next, what we have
compared with the leaves of the books — the divided
portions of the cells — become separated front the backs —
the undivided portions (i>®)— and each leaf (il/. PI) fuses
with the ('orresponding leaves of a certain number of books
in the same longitudinal series. TIte final result is that the
undivided portions of the cells (backs of the books, Ca^L
Eptkm) become the parietal layer of coilomic epithelium, the
longitudinal bands formed by the union of the leaves
(//. PI) becoming the muscle-plates, which are thus ceil-
fusionsy each being formed by the union of portions of a
series of longitudinally arranged cells.
At the same time the cells of the splanchnic layer
of mesoderm thin out and become the visceral layer of
ceelomic epithelium
We see then that by the time the larva has reached the
stage shown in Fig. 74, it is no longer a mere aggregate of
simple cells arranged in certain layers. The cells them-
selves have undergone differentiation, some becoming modi-
fied into nerve-fibres, others by division and subsequent
fusion with their neighbours forming muscle-plates, while
others, such as the epithelial cells, remain almost unaltered.
'rhus, in the course of the development of Polygordius,
cell-multiplication and cell-differentiation go hand in hand,
the result being the formation of those complex tissues the
presence of which forms so striking a difference between the
worm and the simpler types previously studied.
XXV SIGNIFICANCE OF DEVELOPMENTAL STAGES 303
It is important to notice that this comparatively complex
animal is in one stage of its existence the oos{)erm - as
simple as an Amoeba ; in another the polyplast- it is com-
parable to a Pandorina» and in a third the blastula “to a
Volvox ; in a fourth — the gastrula — it corresponds in general
features with a Hydra; while in a fifth — the trochosphere - -
it resembles in many respects a Medusa. As in other cases
we have met -with, the comparatively highly-organised form
passes through stages in the course of its individual develop-
ment similar in general characters to those which, on tlu‘
theory of evolution, its ancestors may be considered to have
passed through in their gradual ascent from a lower to a
higher stage of organization.
The rest of the development of Polygordius may be
summarized very briefly. The trunk grows so much faster
than the head (pro-^/us peri-stomium) — that the latter under-
goes a relative diminution in size, finally becoming of equal
diameter with the trunk, as in the adult. 'The ciliated rings
are lost, the tentacles grow to their full size, the eye-spots
atrophy, and thus the adult form is assumed.
LESSON XXVI
TflE CHIEF DIVISIONS OF THE ANIMAL KINGDOM : THE
STARFISH
The student who has once thoroughly grasped the facts of
structure of such typical unicellular animals as Amceba and
the Infusoria, of such typical diploblastic animals as Hydra
and Bougainvillea, and of such a typical triploblastic animal
as Polygordius, ought to have no difficulty in understanding
the general features of the organization of any other members
of the animal kingdom. When once the notions of a cell, a
cell-layer, a tissue, an organ, body-wall, enteron, stomodaium,
proctodaeum, cielome, somatic and splanchnic mesoderm,
are fairly understood, all other points of structure become
hardly more than matters ot detail.
If we turn to a text-book of Zoology we shall find that
the animal kingdom is roughly divisible into eight primary
sub-divisions, called sub-kingdoms, types, or phyla. These
are as follows : —
Protozoa.
Porifera,
Ctr/en/erata.
“ FermesJ*
Ecki nodermata,
Arihropoda.
Mollusca.
Fertehraia.
LESS. XXVI GENERAL STRUCTURE 305
With a few exceptions, the discussion of which would be out
of place here, the vast number of animals known to us may
be arranged in one or other of these groups.
The Protozoa are animals which are either unicellular in
the strict sense, or non-cellular, or colonies of unicellular
zooids : they have been represented in previous lessons by
Amoeba and Protamueba, Hjematococcus, Heteromita,
Euglena, the Myceto/.oa, Paraimecium, Stylonychia, Oxy-
tricha, Opalina, Vorticella, Zoothamnium, tlie J'oraminifera,
the Radiolaria, Pandorina, and Volvox. T'he reader will
therefore have no difficulty in grasping the general features
of this phylum.
The Calenterata are the diploblastic animals, and have
also been well represented in the foregoing i)ages, namely
by Hydra, Bougainvillea, Diphyes, and Porpila. 'Phe sea-
anemones and corals also belong to this phylum, in which
also the Porifcra (jr sponges were formerly included.
T'he VermeSy' or Worms, are a very heterogeneous assem-
blage. They are all triploblastic, but while some are
coelomate, others have no body-cavity ; some, again, are
segmented, others not. Still, if the structure of Polygordius
is thoroughly understood, there will be little difficulty in
understanding that of a fluke, a tape-worm, a round-worm,
an earthworm, or one of the ordinary marine worms.
Of the remaining four sub-kingdoms we have, so far,
studied no example, but a brief description of a single
example of each will show how they all conform to the
general plan of organi.sation of Polygordius, being all triplo-
blastic and coelomate.
Under the Echinodertnata are included the various kinds
of starfishes — sand-stars, brittle-stars, and feather-stars, as
well as sea-urchins, .sea-cucumbers, &c. A starfish will serve
as an example of the group.
X
3o6
THK STAKKISH
I^ESS.
'riic {)hyUini Art/iropoda includes crayfishes, lobsters,
crabs, shrimps, prawns, wood-lice, and water-fleas ; scorpions,
spiders, and mites ; centi{)edes and millipedes ; and all
kinds of insects, such as cockroaches, beetles, flies, ants,
bees, butterflies, and moths. A crayfish forms a very fair
example of the acpiatic kinds (Cntsfacec^*
In the phylum Alollusai are included the ordinary bi-
valves, sucli as mussels and oysters ; snails, slugs, and other
univalv(*s or one-shelled forms ; and cuttle-fishes, squids, and
Octopi. An account of a fresh water mussel will serve to
give a general notion of the character of this group.
k’inally, under the head of Vcrfelfrata are included all the
backboned animals : the lam})reys and hags : true fishes,
such as the shark, skate, sturgeon, cod, perc^h, trout, &:c. ;
amphibians, such as frogs, toads, newts, and salamanders ;
true reptiles, siu'h as li/ards, crocodiles, snakes, and tor-
toises ; birds : and mammals, or creatures with a hairy skin
which suckle their young, such as the ordinary hairy
quadrujieds, whales and porpoises, a{)es, and man. ^I'he
essential structure of a vertebrate animal will be understood
from a brief description of a dog fish.
The Starfish.
The commonest British starfish is Asfenas ruhens^ but
the main features of the following description ^^ill apply to
any species. The starfish consists of a central disc-like
portion, from which radiate five arms or rays, 'Fhe animal
crawls over the rocks with its flat, light-coloured ventral
surfac'e downwards, and with its darker, convex, dorsal
surface upwards. It can move in any direction, so that, in
the ordinary sense of the w’ords, anterior and posterior ex-
tremities cannot be distinguished. Radial symmetry such
XXVI
TUBE-FEET
307
as this, />., the division of the body into similar parts
radiating from a common centre, is characteristic of the
Echinodermata generally.
In the centre of the disc on the ventral surface is a five-
sided depression, at the bottom of which is the large mouth
(Fig. 75 and Fig. 76, a, Afth). From it radiate five grooves
Fig. 75. — A Starfish, from the ventral aspect, sliowinj^ the disc and
arms, the central immth, and the numerous tuhe-feel, (From Barker
and Uasweirs after Leuckart and Nitsche. )
called the amlnilacral y^rooves, one along the ventral surface
of each arm (P 1 g. 76, a and h). In the living animal numerous
delicate semi-transparent cylinders, the iuhe-feet (Fig. 75 and
protruded from these grooves ; they are
very extensible and each ends in a sucker It is by moving
these structures in various directions, protruding some and
withdrawing others, that the starfish is able to move along
X 2
3o8
THE STARFISH
LESS. XXVI
either a horizontal or a vertical surface, and even to turn
itself over when placed with the ventral side u{)wards.
Near the middle of the disc, on the dorsal surface, is llie
very minute anus (Fig. 76 a, An ) ; it is situated on a line drawi^
from the centre of the disc to the re-entering angle between
two of the rays, and is therefore said to be inter-radial in
position. Near the anus, and also inter-radially situated, is
a circular calcareous plate, the madreporite i^Mdpr\ per-
forated by numerous microscopic ai>ertures. 'Fhe presence
of this structure disturbs the radial symmetry of the starfish
and gives rise to a bilateral symmetry, since the animal can
be divided into two truly equal halves by a single plane
only, viz,^ the plane passing through the middle of llie
madreporite and of the arm opposite to it.
'I’he body, though flexible, is tolerably firm and resistant,
owing to the fact that immediately beneath the soft, slimy
skin there is a layer of little irregular calcareous bodies, the
ossicles (Fig. 76, os\ forming a kind of scale armour. Many
of them give attachment to spines, and between them are
minute apertures, the dermal pores^ through which, during
the life of the animal, are protruded delicate, glove-finger-
like processes, the dermal gills or respiratory cceca {jResp,
C(e), Both on the dorsal and the ventral surfaces are found
curious and characteristic organs called pedicellariie {Ped\
I'hese are minute forceps-like structures, consisting of a
basal piece or stalk and of two jaws, each supported by a
calcareous plate : the jaws are worked by muscles, and
apparently serve to remove fsecal matter, foreign bodies, &c.,
from the surface of the animal.
The tube-feet, already referred to, are arranged symme-
trically on either side of each ambulacral groove. At the
extremity of the groove is a single structure {t) like a tube-
foot without the terminal sucker : it is called the tentacle^
A
Fig. 76, — Diagrammatic sections of a Slaifish.
A, vertical section passing on the right through a radius, on the left
through an inter-radius. The off-side of the ambiilacral groove, with
the tube feet {T. F) and ampulla* {Amp)^ is shown in perspective.
B, transverse section through an arm.
The ectoderm is coarsely dotted, the nervous system finely doited, the
endoderm radially striated, the mesoderm evenly shaded, the ossicles of
the skeleton black, and the coelomic epithelium represented by a beaded
line.
'I'hc body-wall consists of deric epithelium {Der, Epthm), dermis
{Da'oi), and the parietal layer of ccelomic epithelium [Coi. Eplhm).
To the body-wall are attached pedicellaria {Ved)^ and the end of tlic
arm bears a tentacle (/) with an ocellus (oc) at its base.
The .skeleton consists of os.sicles (cfs) imbedded in the dermis : large
ambulacral ossicles {Amb. as) bound the ambulacral grooves on the
ventral surfaces of the arms.
The mouth {A/Fi) leads by a short gullet into a stomach (.9/), which
gives off a cardiac caecum {Cd. ca) and a pair of pyloric caeca {Jy/. C(x)
to each arm, and pa.sses into an intestine (/«/) which gives off intestinal
cieca (/;//. ae) to the inter-radii, and ends in the anus (An), The
pyloric caeca are connected to the dorsal body-wall by mesenteiies
{Afes. in B). The wall of the enteric canal consists of enteric epithelium
covered by the visceral layer of cnelomic epithelium (C<r/. Epthin^).
From the coelome are given off respiratory caeca {Resp. which
project through the body-wall : the latter contains spaces {p. h) derived
from the coelome.
310
THE STARFISH
LESS.
The circular blood-vessel {C, B. V) surrounds the gullet and give*,
off radial vessels (AW, B. V) to the arms and an inter-radial plexus
connected with a pentagonal ring round the intestine.
The circular anibulacral vessel (C. A mb. V) gives off radial vessels
(AW. Amh. V) to the arms connccte<l with the ampullre {Amp) and
tube-feet ( F ) : it is also connected with the stone-canal {St. C\ which
opens externally by the madreporite {Mdpr).
The nerve ring {Nv, R) gives off radial nerves {Rad. Nv) to the
arms.
The ovary {Ovy) is inter-radial, and opens by a dorsal oviduct {Ovd).
and is proljably an organ of smell. At the base of the
tentax'le is a bright red eye-spot {pc).
Sections show that there is a well-marked ctelome,
separating the body-wall from the enteric canal and contain-
ing the gonads, blood-vessels, ^cc. The body- wall consists
externally of a very thin cuticle, then of a layer of dene
epithelium or epidermis {Der. Epthm), then of a thick,
double, fibrous layer {Derm), then of a thin and interrupted
layer of muscle, and finally, of a layer of cuelomic epithelium
( Ciii. Epthm) bounding the body- cavity.
T’he ossicles with their spines together form an external
skeleton or exoskekton : as already mentioned they are, for
the most {)art, small irregular bodies developed in the
fibrous layer of the body-wall, and overlapping one another
in a scale-like fashion. But the ambulacral grooves are
bounded by regularly arranged {xiirs of large, rod-like aniku-
iacral ossicles {A mb. os\ arranged like rafters, the dorsal
ends of each pair uniting at the summit of the groove,
while their ventral ends diverge and are connected with the
ordinary ossicles at the edge of the arm. Between each
ambulacral ossicle and its predecessor and successor in the
row is an aperture, the amhuhicral pore, with which one of
the tube-feet is connected.
The mouth (Fig. 76, a, mth) leads by a short gullet into
a stofnach {st) divisible into two portions, called respectively
XXVI DIGESTIVE ORGANS 311
the cardiac and pyloric divisions. Thv. cardiac division
(Fig. 77, card. sf)y into which the gullet opens, is a spacious
sac, produced into five wide pouches, the cardiac cara
(Elg. 76, A, ai. ca^ ; Fig. 77), one of which extends into the
Fig. 77. - fjr^ans of a Starfish {J^/cr/<is seen from
the dorsal aspect.
The cardiac portion of the stomach Uard. \l) gives off five shoit
cardiac cieca or pouches and leads into the pyloric division {pyl. v/),
from which five bifid pyloric creca {pyl. Cfic) are continued to the ends
of the arms. The short intestine is recognisable liy the presence of the
intestinal ca?ca p'/;/. (ur) and of the anus (af/) : mad) ,, madreporite,
(From Parker and HaswelPs Zoology, after Leuckarl.)
base of each arm. When the starfish is feeding it can evert
this cardiac sac over the shellfish or other object serving as
prey, and is thus able to devour animals too large to be
taken into the mouth : the everted stomach is afterwards
drawn back by means of special muscles. Dorsajly the
312
TlfK STARFISH
LKSS.
cardiac communicates with the small pyloric division
77> Py^' which also gives off five pouches, the
Pyloric caeca (Fig. 76 and > but each of these,
instead of extending merely into the base of the correspond-
ing arm, divides into two, and both branches extend to the
extremity of the arm, giving off as they go small side-
branches, so that the whole caecum has a tufted or sacculated
character. 'Fhe pyloric caeca are lined by gland-cell.s, and
in them the digestion of the food takes place. They arc
connected with the dorsal walls of the arms by mesenteries
(Fig. 76, B, was).
The pyloric division of the stomach leads into a very
short intestine which passes upwards in a straight line to the
anus (an\ previously giving off two intestinal cteca {int, cue)
situated inter-radially— not radially like the blind offshoots
of both divisions of the stomach.
1'he whole enteric canal is lined with enteric epithelium
(Fig. 76, Ent, Epthtn\ and is covered by the visceral layer
of ccelomic epithelium (CaeL Ept/itn) : it has no muscular
layer. Inhere is a spacious coelome (Gi?/) between the body-
wall and the enteric canal filled with a watery fluid contain-
ing leucocytes. The ctclomic epithelium is ciliated, the
cilia effecting a circulation of the ccjelomic fluid. The
dermal gills {Resp, cce\ already referred to, communicate
with the coelome, and are, in fact, hollow outpushings of
the body-wall They serve to bring the coelomic fluid into
close relation with the surrounding water, and are therefore
to be looked upon as orf^ans of respiration.
One of the most characteristic structures in the anatomy
of the starfish is a peculiar system of vessels called the
water-vasaiiar or amhulacral system: it is of great func
tional importance, being connected with the working of the
tube-feet
XXVI
AMBULACRAL SYSTEM
313
The central part of the ambulacral system is a f)entagonal
tube (Fig. 78, c ; Fig. 76, C. Amb. V) which surrounds the
gullet, and is called the ambulacral ring-vessel. From each
angle of the pentagon is given off a radial ambulacral vessel
Fig. 78. -- The water vascular system of a Starfish (cliagramatic).
The ring- vessel (c) gives off five radial vessels (r), lateral off-shoots of
which (/) are connected with the tube-feet {p) and ampulhe («).
Inter-radially the ring-vessels give off Polian vesicles {ap) and the
madreporic canal {m') ending in the madreporite (w). (From
Gegenbaur. )
(Fig. 78, r ; F'ig. 76, Rad. Amb. V) which proceeds to the
end of the corresponding arm, lying in the dihedral angle
included by the double row of ambulacral ossicle.s, and
consequently external to this portion of the skeleton (Fig. 76,
b). Each radial vessel sends off side branches (Fig. 78, r)
3*4
VHK STARFISH
LESS.
which communicate with the hollow tube-feet (P'ig. 78, / ;
P'ig. 76, T. /:), and each tube-foot is connected by a narrow
canal passing through an ambulacral pore (p. 310) with a
bladder-like body, the ampulla (P'ig. 78, a ; P'ig. 76, Amp)
lying in the ccelome. 'Fhe ampullae consequently form a
double row of bladders along the ventral region of the
interior of the arm.
1’he ring-vessel also gives off inler-radially, />., in the
intervals between the arms, bladder-like bodies, the Polian
vesicles ( P"ig. 78, ap)^ one or two in each inter-radius. In one
of the inter-radii there also goes off from the ring-vessel a tube,
('ailed the stone-canal (P'ig. 78, ni : P'ig. 76, St. c) from the
fact that its walls are calcified, which passes directly upwards
and becomes connected with the madreporite (Fig. 78, m ;
Fig. 76, A, Mdpr). 'The latter is perforated by minute
apertures which are in communication with the cavity of the
stone-canal, and in this way the ambulacral system is placed
ill direct communication with the surrounding water.
I'he whole ambulacral system contains a watery fluid, and
its w'alls ('onsist of a lining of epithelium and an outer
muscular layer particularly well developed in the ampullae
and tube-feet. Contraction of the muscles of the ampullae
forces water into the tube-feet, and causes protrusion of
these organs : their withdrawal is brought about by the con-
traction of the longitudinal muscles in their walls, by which
the fluid is forced back into the ampullae.
Thus the whole ambulacral system forms an elaborate
locomotory apparatus worked by water-power. It is quite
confined to lilchinoderms. In all the other higher animals
movements are effected by the direct, and not, as in this
case, by the indirect action of muscles.
A second system of vessels constitutes the so-called
blood-system. Surrounding the gullet below the ambulacral
XXVI REPRODUCTIVE ORCIANS 315
ling-vessel is a ring blood-vessel (Fig. 76, a, C. B, V), send-
ing off radial blood-vessels {^Rad. B. V) to tlie arms. An
inter-radial shins or blood-space lies alongside the stone
canal, surrounding the ovoid gland (see j). 316), and is con-
nected below with the ring-vessel and above with a
pentagonal vessel or sinus, from which inter-radial branches
proceed to the gonads.
The nervous system is considerably simpler than that of
Polygordius. It consists, in the first place, of a pentagonal
nerve-ring (Pig. 76, a, Nv^ R) surrounding the mouth, and
having the character of a mere thickening of the deric
epithelium. From each of its angles goes off a radial fieri'e
{Rad. Nv) which passes along the arm below the ambu-
lacral and blood-vessels, and is also nothing more than a
thickening 6f the epidermis, some of the cells of which are
modified into nerve-cells and fibres. At the end of the
arm the radial nerve terminates in the eye-spot. In addition
to this superficial nervous system there is a deep nervous
system, situated internally to the former, and consisting of a
double jientagon round the mouth, sending off double radial
nerves to the arms, d'here are also scattered nervous
elements in the dorsal region of the body-wall.
Like Polygordius, the starfish is dia^cious : tliere is no
external distinction between the sexes, and even the ovaries
and spermaries can be distinguished only by microscopic'al
examination. There are five pairs of gonads ovaries
(Pig. 76, A, ovy) or spermaries as the case may be one
pair in each inter-radius. Each gonad has the form of a
bunch of grapes, being a much-lobed sac lined by epithelium
from which the ova or sjxirms are developed. It is con-
tinued into a tube or gonoduct, ('ailed spermiduct in the
male, oviduct {Ovd) in the female, which opens inter-radially
on the dorsal surface close to the bases of th<3 arms. The
THK STAKMSn
LESS,
316
gonadh arc all connected by cords of tissue with an organ
called the axial or^an, which lies alongside the stone canal
and is surrounded by a blood-sinus. Its function is not
known with certainty.
The ova and sperms are shed into the water, where im-
Fio. 70. -Knily stages in lln* deM'lojMnenl of n Starfish.
A. 'I'he po])[)l;ist, surroun(tc(l by the vitelline meinbiaue.
B. The blastula, in section.
c. The gasinila, external \le\^, duiwing the blastopore {dl. />).
i> The g.istrnla, in vertical secnon ; enleron.
1 Mou- advanced gastrula, with ciliated ectoderm.
.1) /'/, enieicn ; Hasfoc, blast occcle ; N.p. blastopore ; cct^ ectoderm ;
I’uJ. endodenii.
(loom r.irker .and llaswell’s Zoolog}>.)
prc'gnation takes place. The oosperm undergoes the usual
proct'ss <d si'gnicntation, forming a polyplast (Fig. 79, a),
\vhi( h ts soon con\crt<'(l int<^ a blastula (b) by the c<>lls arrang-
ing theinsc!v('s round central cavity. One -ah' ol the
blastula becomes invaginated or tucked in, and a gastrula
XXVI
DEVKLOrMKNT
.it7
(c, I), k) is formed, the t'rlls lu'c-ommL; didcicnlialed uilo
t'('t()derm and endodoim, and tlu‘ ci’lodei m (( IK ac<juiiini;
cilia, 'riu' ga.slrula gradually takes on tlu* form ol a pnailiar
free -swimming larva having a certain geneial lesi'inhlance
to the trochos})herc‘ and called a bi/>ifuiiu in (Mg. So): it
differs from the adult starfish in showing no tract' ol radial
symmetr\, the body being jiroduct'd into si'veral cilialt'd
Fig. 8o.~ 'fhice stages in the development of the Bi])inri:u‘ia larva of
a Starfish. .///, anus; pre-oral ciliated ring; ino, uioulli ;
post-oral ciliated ring. (From Tarkcr and Ilaswell, after Leueknrt
and Nitsche.)
processes or arms, all bilaterally arranged, and the enteric
canal having the form of a curved cylindrical tube, consist-
ing of gullet, stomach, and intestine lying in the median
plane. 1'hc bipinnaria lives a free life for a time, swiimnmg
by means of it^ cilia, and finally, by a complex series of
changes, undergoes gradual metamorphosis into the adult
starfish.
LESSON XXVII
THK CRAYFISH
'rnK Starfish has furnished us with an example of an
animal in which an obvious radial symmetry is, as it were,
superposed upon an original bilateral symmetry : in which
also there is an extremely simple form of nervous system,
a unique type of locomotory apparatus, and no trace of
metameric segmentation. \Xc have now to study, in the
crayfish, an animal forn\ed upon quite the same general
plan of structure as Polygordius as to segmentation, arrange-
ment of organs, c\:c,, but which reaches, in every respect, a
far liigher grade of organisation,
'I’he C'ommon British Fresh-water Oayfish is Astacus
fiuvialilis : allied species occur in Europe, Asia, and
America. 'Phe following description will apply almost
e(|ually well to the l^obster, Homarus vitl^i^aris.
'The body of the crayfish (Fig. 8i) is divided into two
regions, an anterior, the cephalothorax^ which is unjointed
and is covered by a cuirass-like structure, the carapace, and
a posterior, the abdomen, which is divided into distinct seg-
ments, movable upon one another in a vertical plane. The
cephalothorax is again divided into two regions, an anterior,
the head (cth\ and a posterior, the thorax (hd), by a trans-
I, ESS. xxvn K\i Ki\NAl ( I1A1\\< 11 Ks
vt'fsi* <i( |)U“^su>ii, the u'rvh’it/ i^yoovc. 1 Ik' (Mmikuc is
LlfM'lojkd Irotn iIumIoixiI and lateral n^i^inns ol hnlti head
anti thorax : it is liee at the bides ot the thonix, wheie it
Fig. Sl. — Siile view of r n ii v, '1< i ' i '\li fi, ii ilui.il si/c.
The cephnlothoiav is co\( K (i 1)\ ili' < . piu.iiK m| m (kjiiI ini
a roslium (/•) and iiilo ecjihalic {c(h] and thoia( ir ikd) |i(>tUon
separated ijy an oMniuo ciuieal i^ioovc. ’! he line from kt! points t
the gill-cover.
The abdomen (<z/') is made up ol n 1\ moxal.l} nilieulalid ipintiit
(xiv-xix', fnlloued by a tel^m, the cxliennly <>l uhidi is indieaicil !>
the lovur end oi the bon l.<'f fiom ah,
't he eve slalk is seen at the ba-.< of the rostrum.
( )f the et [dialie appendage^ llie .niO nmile oz' and niiUmia Or ai
shovMi ; of the thoracic aj'|jend.ige the ihiol iii.ivillip' <1 (h , liie > ulaigi'
first leg tu iheliped(9'. :in<! lie hmi sh nd* r walling hg.iio <
the abdominal apjiendagi > three plc‘oj>o(K and llu- uiopod ' iSj.
(Krom Lang, after Huxley. j
forms a fltip or (Fig. 83, u, on oath sidi
sc'paratetl from tin* a< Itiul hodv wall h\ ;t naroiw spnoc i
which the gills are < otii.imi d
THE CRAYFISH
or appendageSy structures which we have not liitherto met
with. Both trunk and ap[x:ndages are covered with a sort
of shell, formed of a substance called chitiny strongly im-
pregnated with carbonate of ’*me so as to be hard and but
slightly elastic.
The abdomen is made up of seven segments : the first
six of these (Fig. 8i, xiv-*xix) are to be considered as meta-
meres in the sense in which the word is used in the case of
i^olygordius. Each has a ring-like form, presenting a broad
tlorsal region or tergum ; a narrow ventral region or sternum ,
and downwardly directed lateral processes, the pleura. The
seventh division of the abdomen is the telson : it is flattened
horizontally and divided by a transverse groove into anterior
and posterior portions. All seven segments are calcified,
and are united to one another by chitinous articular mem-
branes : the first segment is similarly joined to the thorax.
Thus the exoskeleton of the Crayfish is a continuous
structure, but is discontinuously calcified so as to have the
character of a hard jointed armour.
It has been stated that the abdominal segments are
movable upon one another in a vertical plane, z>., the whole
abdomen can be extended or straightened, and flexed or bent
under the cephalothorax : the segments are incapable of
movement from side to side. This is due to the fact that,
while adjacent segments are connected dorsally and ven-
trally by flexible articular membranes, they present at each
side a joints placed at the junction of the tergum and
pleuron, and formed by a little peg-like process of one seg-
ment fitting into a depression or socket in the other. A
line drawn between the right and left joints constitutes the
axis of artietdationy and the only possible movement is in a
plane at right angles to this axis.
Owing to the presence of the carapace the thoracic region
3 CXVII
APPENDAGES
321
is immovable, and shows no distinction into segments cither
on its dorsal (tergal) or lateral (pleural) aspt'ct. But on the
ventral surface the sterna of the thoracic segments are
clearly marked off by transverse grooves, and the hindmost
of them is slightly movable Altogether eight thoracic
segments can be counted.
The ventral and lateral regions of the thoracic exoskeleton
are produced into the interior of the body in the form of
a segmental series of calcified plates, so arranged as to form
a row of lateral chambers in which the muscles of the limbs
lie, and a median tunnei-like passage or sternal canal^ con-
taining the thoracic portion of the nervous system. 'I’he
entire endophragmal system^ as it is called, constitutes a kind
of internal skeleton (Fig. 83, h).
The head exhibits no segmentation : its sternal region is
formed largely by a shield-shaped plate, the episfoma, nearly
vertical in position. The ventral surface of the head is, in
fact, bent so as to face forwards instead of downwards. I'he
cephalic region of the carapace is produced in front into a
large median spine, the rostruni (Fig. 81, r) : immediately
below it is a plate from which spring two movably articu-
lated cylindrical bodies, the eye-stalks^ bearing the eyes at
their ends.
The appendages have very various forms, and are all, like
the abdomen, jointed or segmented, being divisible into
freely articulated limb-segments or podomeres. The observer
is at once struck by the long feelers attached to the head, the
five pairs of legs springing from the thorax, and the little
fin-like bodies arising from the sterna of the abdomen. Jt
will be convenient to begin with the last-named region.
The third, fourth, and fifth segments of the abdomen
bear each a pair of small appendages, the swimming-feet
or pleopods, A pleopod (Fig. 82, 10) consists of an axis or
Y
7 .
g(^0^y54tory Orga/iiS
itUrofJod
Fic. 82."-The principal appendages cf the iMcsli-water Crayfish
placed in (he same [K)siti<Mi, with the pioloixidite /’; ) and epipodite [ep]
downwaids, the endnpndilc ti> llic left, and the exopodite (^’v) tt>
the right.
The protopoditc is typical!) formed of two podonicics [pr, i, pr. 2],
the endopoditc of fne (c//, i cn, 5I : a gill {:,■) m:i) be attached to the
epipodite and a hunch of long set.e to the protopoditc t7 and 8).
The three segments of the anlennide are marked 13, its flagella /. l
and fi. l \ at the distal end of the endopodite of the antenna is a
flagellum (//).
(From Farkerand HasweH’s after Huxley.)
xxvii APPENDAGES 323
protopodite having a very short proximal (/r. i), and a long
distal {pr, 2) podomcre, and bearing at its free end two
jointed plates, fringed with setie, the endopodite (en) and
exopodite (ex). These appendages act as fins, moving back-
wards and forwards with a regular swing, and probably aid-
ing in the animafs forward movements.
In the female a similar apixiiidage is borne on the second
segment, while that of the first is more or less rudimentary.
In the male the first and second pleopods (9) are modified
into incomplete tubes which act as copulatory organs, serving
to transfer the spermatophores to the body of the female.
The sixth pair of pleopods (ii) are alike in the two sexes:
they are very large, both endo- and exopodite having the
form of broad fiat plates : in the natural position of the
parts they lie one on each side of the telson, forming with
it a large five-lobed tail fin : they are therefore conveniently
called uropods or tail-feet. The telson itself bears no
appendages.
The thoracic appendages are very different. I'he four
posterior segments bear long slender, jointed kgs (Fig. 81,
8), upon which the animal walks : in front of these is a pair
of very large legs terminating in huge claws or chelce^ and
hence called chelipeds. The three anterior segments bear
much smaller appendages (6, 7) more or less leg-like in
form, but having their bases toothed to serve as jaws : they
are distinguished as maxillipeds or foot-jaws.
The structure of these appendages is best understood by
a consideration of the third maxilliped (Fig. 82, 7). I'he
main portion of the limb is formed of seven podomeres
arranged in a single series, strongly calcified, and, with the
exception of the second and third, which are fused, movably
articulated with one another. The second podomere,
counting from the proximal end, bears a many-jointed
Y 2
324
THE CRAYFISH
J.ESS.
feeler-like organ (ex)^ and fioin the first springs a thin, folded
plate (ef) having a plume like gill {g) attached to it. The
first two segments of the axis form the protopodite, its
remaining five segments the endopodite, and the feeler,
which is directed outwards, or away from the median plane,
tlie exoi)odite. The folded plate is called the epipodite : in
the natural position of tlie parts it is directed upwards, and
lies in the gill-cavity between the proper wall of the thorax
and the gill-cover (k'ig. 87, a, /M).
T’he five legs (8) differ from the third maxilliped in cheir
greater size, and in having no exopodite : in the fifth or last
the epipodite also is absent. The first three of them have
undergone a curious modification, by which their ends are
converted into pincers or the fourth segment of the
endopodite (sixth of the entire limb, e 7 i. 4) is produced dis-
tally so as to form a claw-like projection {en, 4'), against
which the terminal segment {en, 5) bites. The first leg is
much stouter than any of the others, and its chela is of
immense size, and forms an important weapon of offence
and defence. I’he second maxilliped resembles the third,
but is considerably smaller : the first (6) has its endopodite
greatly reduced, the two segments of its protopodite large
and leaf-like, and no gill is connected with the ei)ipodite.
The head bears a pair of mandibles and two pairs of
maxilla; in relation with the mouth, and in front of that
aperture a pair of antennules and one of antennae. The
hindmost appendage of the head is the second maxilla (5),
a leaf-like appendage, its protopodite being cut up into
lobes, while the exojKidite is modified into a boomerang-
shaped plate, which by its movements produces a current of
water over the gills. The first maxilla (4) is a very
small organ, having neither exo- nor epipodite. 'Fhe man-
dible (3) is a large, strongly calcified body, toothed along
xxvn
APPENDAGES
325
its inner edge, and bearing on its anterior bordt r a little
three-jointed feeler-like body, the palp^ the two distal seg-
ments of which represent the endopodite, its proximal
segment, together with the mandible proper, the protopodite.
The antenna (2) is of great size, being nearly as long as
the whole body. It consists of an axis of live podomeres,
the fifth or last of which bears a long, flexible, many-jointed
structure, or flagelhim (yf), while from the second segment
springs a scale-like body or sguame {ex). It is fairly obvious
that the two proximal segments represent the protopodite,
the remaining three, with the flagellum, the endopodite, and
the squame the exopodite.
The antennule (i) has an axis of three podomeres ending
in two many-jointed flagella {fl, i,yf. 2), which are some-
times considered as endo- and exopodite. But in all the
other limbs, as we have seen, the exopodite springs from
the second segment of the axis, and the jirobabilities are
that there is no exact correspondence between the parts of
the antennule and those of the remaining appendages.
The eye-stalks^ already noticed, arise just above the an-
tennules, and are formed each of a small proximal and a
large distal segment. They are sometimes counted as
appendages serially homologous with the antennae and
legs, &:c., but are more properly to be looked upon as
articulated processes of the prostomium. It is possible
that the antennules are also prostomial and not metameric
structures : assuming this to be the case, it will be seen
that the body of the crayfish consists of a prostomium,
eighteen metameres, and a telson, which is probably com-
posed of an anal segment plus a post-anal extension. The
prostomium bears eye-stalks and antennules : the first four
metameres are fused with the prostomium to form the head,
and bear the antennae, mandibles, first maxillae, and second
THE CRAYFISH
LESS.
326
maxillae : the next eight metameres (fifth — twelfth) consti-
tute the thorax, and bear the three pairs of maxillipeds and
the five pairs of legs : the remaining six metameres (thirteenth
— eighteenth), together with the anal segment, constitute
the abdomen, and bear five pairs of pleopods and one of
uropods.
The articulation of the various podomeres of the append-
ages is on the same plan as that of the abdominal segments
(p. 320). The podomeres are, it must be remembered, rigid
tubes : they are connected with one another by flexible
articular membranes (Fig. 85, art, m\ but at two points the
adjacent ends of ■ the tubes come into contact with one
another and are articulated by peg-and-socket joints {h\ the
two joints being at opposite ends of a diameter which forms
the axis of articulation, 'Fhe two podomeres can therefore
be moved upon one another in a plane at right angles to
the axis of articulation and in no other direction, the joints
being pure hinge-joints. As a rule the range of movement
is from the perpendicular to a tolerably extensive flexion on
one side — the articulations are single-jointed, like our own
elbows and knees. The whole limb is, however, capable of
universal movement, owing to the fact that the axes of articu-
lation vary in direction in successive joints: the first
joint of a limb bending, for instance, up and down, the
next backwards and forwards, the next obliquely, and so on.
In some ca.ses, e,g,^ in the pleopods, peg-and-socket joints are
absent, the articulation being formed merely by an annular
articular membrane, movement being therefore p)ossible in
any plane.
Sections show the body-wall to consist of a layer of deric
epithelium (Fig. 83, Der, Epfhm) secreting a thick cuticle
(C/.), a layer of connective tissue forming the dermis
(Derm)y and a very thick layer of large and complicated
XXVII
MUSCULAR SYSTEM
327
muscles ( J/), which fill up a great part of the interior of the
body. Neither on the deric epithelium nor elsewhere are
there any cilia, the absence of these structures being gene-
rally characteristic of Arthropods.
The cuticle {Cu) is of great thickness, and except at the
joints between the various segments of the body and limbs,
is impregnated with lime-salts so as to form a hard, jointed
armour. It thus constitutes a skeleton which, unlike that
of the starfish (p. 310), is a cuticular exoskeleton, forming a
continuous investment over the whole body but discon-
tinuously calcified. It is shed and renewed periodically —
once a year during adult life — the process being known as
ecdysis,
'The muscular system shows a great advance in complexity
over that of Polygordius, and consists entirely of transversely
striated fibres. In the abdomen the muscles are of great
size, and are divisible into a smaller dorsal and a larger
ventral set. The dorsal muscles (Fig. 86, em ; Fig. 84,
d. m) are paired longitudinal bands, divided into segments
called myomeres, and inserted by connective tissue into the
anterior border of each segment : anteriorly they are trace-
able into the thorax, where they arise from the side-walls of
that region. When these muscles contract they draw the
anterior edge of each tergum under the posterior edge of
its predecessor, and thus extend or straighten the abdomen.
T’he ventral muscles (Fig. 86, /m) arc extraordinarily
complex. Omitting details, there is on each side a wavy
longitudinal band of muscle (Fig. 84, c.m), nearly circular in
section, which sends on a slip {ex) to be inserted into each
segment above the hinge : the contraction of this muscle
must obviously tend to approximate the terga, and .so aid
the dorsal muscles in extending the abdomen. Around this
central muscle is wrapped, in each segment, a band of
LESS. XX\ II
MUSCULAR SVSTEM
329
The body is divided into a head (///) and thorax (77/), together
constituting the cephalothorax {C. 77/), aii'.l seven free abdominal
segments {Add, seg. i, AM, seg, 7 ) : the head is produceil in front into
a rostrum ( A^).
The body-w'all consists of cuticle (C//), partly calcified to form the
exoskelcton, deric epithelium [Dt:r, Epthm\ dermis {Derm\ and a
very thick layer of muscle (J/) which in the abdomen is distinctly
segmented.
The mouth {Mlh) leads by a short gullet {Gul) into a large stomach
(.S7), from which a short small intestine (.S’, /fti) leads into a large in-
testine (Z. /;//), ending in the anus (An), (jpening into the small
intestine are the digestive glands (D, Gl). The epithelium of the small
intestine and digestive glands is endodermal, that of the rest of the canal
is ectodermal and secretes a cuticle : the outer layer throughout is
mesodermal (connective tissue and muscle).
The cavity (B. .S’) between the enteric canal and the body-muscles is
a blood-sinus.
I'he heart (///) is enclosed in the pericardial sinus (Per. .V): the
chief ventral blood-vessel or sternal artery (St. A) is shown in n.
The gills (b. Gill) arc enclosed in a cavity formed by a fold of the
thoracic body-w'all called the branchiostegite (Brst }^) : they are formed
of the same layers as the body-wall, of which they arc offshoots.
1'hc kidneys (A, K) arc situated in the head.
The brain (AV) lies in the prostomium : the ventral nerve-cord (V.
Nv, Cd) consists of a chain of ganglia (Gn) united by conned ives.
The ovary (//r/y) is a hollow organ opening by an oviduct (b, ovd) on
the base of one of the legs (Leg).
muscle {env. m) in the form of a loop, the outer limb of
which (fl) turns forwards and is inserted into a sternum,
wdiile the inner limb {Ji') turns backwards and is inserted
into another and more posterior sternum. The contraction
of this enveloping muscle produces an approximation of the
sterna, and thus flexes the abdomen, the central muscle
always keeping the middle of the loop in place. The
ventral muscles are, like the dorsal, traceable into the
thorax, w^here they arise from the endophragmal system :
their various {xirts are connected by a complex system of
fibres extending betw'een the central and enveloping muscles,
and connecting both with their felIow\s of the opposite side.
The flexor muscles are immensely powerful, and produce,
when acting together, a sudden and violent bending of the
Fig. 85. — A leg of the Fresh-water Crayfish with part of the cxo-
skeleton removed to show the muscles.
en. 2 ~en. 5, segments of endopodite ; V/, hinges ; arf. m, articular
mem brane ; ext^ extensor muscles ; flexor muscles.
(From Parker and llaswell’s Zoology,)
332
THE CRAYFISH
LESS. XXVII
It will he seen that the body-muscles of Astacus cannot
be said to form a layer of the body-wall, as in Polygordius,
but constitute an immense fleshy mass, filling up the greater
part of the body-cavity, and leaving a very small space
around the enteric canal.
In the limbs (Fig. 85) each podomere is acted upon by
two muscles situated in the next proximal podomere. I'hese
muscles are inserted, by chitinous and often calcified
tendons, into the proximal edge of the segment to be
moved, the smaller {ext) on the extensor, the larger (//) on
the flexor side, in each case half-way between the two
hinges, so that a line joining the two muscular insertions is
at right angles to the axis of articulation.
The digestive organs are constructed on the same general
f)lan as those of Polygordius, but present many striking
differences. The mouth (Fig. 83, A, Mth) lies in the middle
ventral line of the head, and is bounded in front by a shield"
shaped process, the hibrutu^ at the sides by the mandibles,
and behind by a pair of delicate lobes, the pamgmitha. It
leads by a short wide gullet (Fig. 83, Gut ; Fig. 86, ee) into
a capacious “ stomach^^^ which occupies a great part of the
interior of the head, and is divided into a large anterior or
cardiac division (Fig. 83, St ; Fig. 86, cs)y and a small pos-
terior or pyloric division (ps) : the latter passes into a narrow
and very short small intestine (Fig. 83, S. Int ; Fig. 86, md)^
from which a somewhat wider large intestine (Fig. 83, Z.
Jnt ; Fig. 86, hd) extends to the anus (ati), situated on the
ventral surface of the telson.
The outer layer of the enteric canal consists of connective
tissue containing striped muscular fibres: within this is a
single layer of columnar epithelial cells, none of them
glandular. In the gullet and stomach, and in the large
intestine, the epithelium secretes a layer of chitin, which
.U<JiX
lie Si gC T t^h mfe«r w-Mk % rtrwing tli#
g^keltt^n wtili ttw wl tl^iiws»el^„ <l%«%twe glmtl iirf
kitlwey erf tbc ughl wet© GmpmtG %ttt| rfi if f tm fe re, 1 1 A|
aoteapaiy «li.f|' » #4 4bd«mcn, I 4
f%ht <lifefti%4 iwet ex.pm44 N glitwl J/ 4 ^ < litli|*«I 4%
fftttst »tm mfi M cai«i»t 4*54*^ ^iiniifi#ii , <14 «piiti»
334
THE CRAYFISH
LESS.
thorax ; rt’, gullet ; em^ dorsal mubclcs ; fm, ventral muscles ; brain ;
//, heait ; hd, large intestine ; /r, left digestive gland ; md^ small intes-
tine ; 0s right lateral ostium of heart ; ophthalmic aitery ; oaUs dorsal
abdominal artery ; «■, gullet ; //. l~5, pleopods ; //. 6, uropod ; ps^
pyloric division of stomach ; s. a, sternal artery ; / (near heart), testis ;
t (below anus) telson ; ttaa^ ventral aMoininal artery ; v. r/, vas defer-
ens ; vdOf male genital aperture.
(From Lang, after Huxley, )
thus constitutes the innermost layer of those cavities. It is
proved by develo])ment that the small intestine, which has
no chitinous lining, is the only part of the enteric canal
develoi)ed from the enteron of the embryo : the gullet and
stomach arise from the stomodeeum, the large intestine from
the proctodieum. T’hus a very small portion of the enteric
epithelium is endodermal (sec Fig. 83, a).
In the cardiac division of the stomach the chitinous
lining is thickened and calcified in certain parts, so as to
form a complex articulated framework, the gastric mill^ on
which are borne a median and two lateral teeth^ strongly
calcified and projecting into the cavity of the stomach.
Two pairs of strong muscles arise from the carapace, and
are inserted into the stomach; when they contract they
move the mill in such a way that the three teeth meet in
the middle line and complete the comminution of the food
begun by the jaws. The separation of the teeth is effected
partly by the elasticity of the mill, partly by delicate muscles
in the walls of the stomach. The pyloric division of the
stomach forms a strainer : its walls are thickened and pro-
duced into numerous setae, which extend quite across the
narrow lumen and prevent the passage of any but finely
divided particles into the intestine. Thus the stomach has
no digestive function, but is merely a masticating and strain-,
ing apparatus. On each side of the cardiac division is
found, at certain seasons of the year, a plano-convex mass
of calcareous matter, the gastroiitk or “ crab’s-eye,”
XXVII
GILLS
335
The digestion of the food, and to some extent the absorp-
tion of tlie digested products, are performed by a pair of
large glands (Fig. 83, D, Gl : Fig. 86, Ir\ lying one on each
side of the stomach and anterior end of the intestine. 'Fhey
are formed of finger-like sacs or cu^ca^ which discharge into
wide ducts opening into the small intestine, and are lined
with glandular epithelium derived from the endoderm of the
embryo. The glands are often called livers, but as the
yellow fluid they secrete digests proteids as well as fat, the
name hepato- pancreas is often applied to them, or they may
be called simply digestive g/ands. 'bhe crayfish is car-
nivorous, its food consisting largely of decaying animal
matter.
The digestive organs and other viscera are surrounded by
a body-cavity, which is in free communication with the
blood-vessels and itself contains blood. This cavity is not
lined by epithelium, and is to be looked upon as an immenst?
blood-sinus, and not as a true coelome.
There are >vell-developed respiratory organs in the form
of gi//s (F 1 g. 83, I}), contained in a narrow branchial
chamber, bounded internally by the proper wall of the
thorax, externally by the gill-cover or pleural region of the
carapace. Each gill consists of a* stem giving off numerous
branchial filaments, so that the whole organ is plume-like.
The filaments are hollow and communicate with two parallel
canals in the stem — an external, the afferent branchial vein^
and an internal, the efferent branchial vein. The gill is to
be considered as an out-pushing of the body-wall, and con-
tains the same layers — a thin layer of chitin externally, then
a single layer of epithelial cells, and beneath this connective
tissue, hollowed out for the blood channels.
According to their point of origin the gills are divisible
into three sets — first, podobranchice or foot-gills (Fig. 87, a,
I'' IG. S7, Two dissections showing the gills of the Fresh-water Crayfish.
In \ the liglit gill-co\er lias been removed, but the gills are undls-
tuiheil : m u the po<l()])iuncbi;e (/./A. in A) arc cut away, and the outer
set of ait)irol>raiichi.c {ar/’^) turned down to show the inner arlhro-
branclii.v (n'/;) and the plciuobianchi.c (//. />).
All the gilK are numbered according to the segment from which they
Sluing, the first thoracic segment being numbered 6, the last 13.
('/>. 5, scaphognathite.
ah. I, ah. 2, abdominal segments ; antennule ; antenna ; 6-8,
maxillipcds ; 9-13, legs ; //. i, first pleopod.
(From l.ang, after Huxley )
LESS. XXVll
CIRCULATOKY 0R(;ANS
337
pdb)^ springing frora the epipodites of the tlioracic appen-
dages, from which they are only partially separable ; secondly,
arthrobranchm or joint-gills (b, arb\ springing from the
articular membranes connecting the thoracic aj)pendages
with the trunk; and thirdly, pleurobro/tcbia, or wall-gills
{pib)y springing from the lateral walls of the thorax, above
the attachment of the appendages. The total number of
gills is eighteen, besides two filaments representing vestigial
or vanishing gills.
The excretory ory;ans differ both in position and in form
from those of Polygordius. There arc no distinct nephridia,
but at the base of each antenna is an organ of a greenish
colour, the antennary or green gland (Fig. 83, a, AT), by
which the function of renal excretion is performed. 'I'lie
gland is cushion-shaped, and contains canals and irregular
spaces lined by glandular epithelium : it discharges its secre-
tion into a thin-walled sac or urinary bladder^ which opens
by a duct on the proximal segment of the antenna. 'Fhe
green glands are to be looked upon as organs of the same
general nature as nephridia.
The circulatory organs are in a high state of development.
The heart (Fig, 83, lit ; Fig. 86, li) is situated in the dorsal
region of the thorax, and is a roughly polygonal muscular
organ pierced by three pairs of apertures or ostia (Fig. 86, 0)^
guarded by valves which open inwards. It is enclosed in a
spacious pericardial sinus (Fig. 83, Fed. S\ which contains
blood. From the heart spring a number of narrow tubes,
called arteries^ which serve to convey the blood to various
parts of the body. At the origin of each artery from the
heart are valves which allow of the flow of blood in one
direction only, viz,^ from the heart to the artery. From the
anterior end of the heart arise five vessels — a median
ophthalmic artery (Fig. 86, <7a), which passes forwards to the
THE CRAYFISH
l.ESS.
c‘yes ; paired aniennary arteries {aa\ going to tlie anten-
nules, antennai, green glands, ^:c., and sending off branches
to the stomach ; and paired hepatic arteries, going to the
digestive glands. T'he posterior end of the heart gives off
two unpaired arteries practically united at their origin, the
dorsal abdominal artery {paa), which passes backwards
above the intestine, sending branches to it and to the dorsal
muscles ; and the large sternal artery (sa), which |)asses
dircf^tly downwards, indifferently to right or left of the
intestine, j)assing between the connectives uniting the third
and fourth thoracic ganglia, and then turns forwards and
runs in the sternal canal, immediately Ixjncath the nerve-
cord, and sends off branches to the legs, jaws, t'^c. At the
point where the sternal artery turns forwards it gives off the
median ventral abdominal artery (y. a. a\ which passes
backwards beneath the nerve-cord, and supplies the ventral
nuis(des, pleopods, <Scc.
All these arteries branch extensively in the various organs
they su])ply, becoming divided into smaller and smaller off-
shoots, which finally end in microscopic vessels called
capillaries These latter end by open mouths which com-
municate with the blood-sinuses, spacious cavities lying
among the muscles and viscera, and all communicating
sooner or later with the sternal sinus (Fig. 83, a, B, S),
a great median canal runmng longitudinally along the
thorax and abdomen, and containing the ventral nerve-cord
and the sternal and ventral abdominal arteries. In the
thorax the sternal sinus (Fig. 88, st, s) sends an offshoot to
each gill in the form of a well-defined vessel, which passes
up the outer side of the gill and is called the afferent
branchial vein (af. br, %i). Spaces in the gill-filaments place
the afferent in communication with the efferent branchial
vein {eff br, v). which occupies the inner side of the gill-
XXVll
CIRCULATION
339
stem. The eighteen efferent branchial veins open into six
branchio-cardiac veins {br. i\ 7/), which pass dorsally in close
contact with the lateral wall of the thorax and open into
the pericardial sinus.
The whole of this system of cavities is full of blood, and
the heart is rhythmically contractile. Wdien it contracts the
blood contained in it is prevented from entering the i)eri-
cardial sinus by the closure of the valves of the ostia, and
therefore takes the only other course open to it, viz.,, into
the arteries. When the heart relaxes, the blood in the
arteries is prevented from regurgitating by the valves at
their origins, and the pressure of blood in the pericardial
sinus forces open the valves of the ostia and so fills the
heart. 'I’hus in virtue of the suc cessive contractions of the
heart, and of the disposition of the valves, the blood is kept
constantly moving in one direction, viz., from the heart by
the arteries to the various organs of the body, where it
nreives carbonic acid and other waste matters ; thence by
sinuses into the great sternal sinus ; from the sternal sinus
by afferent branchial veins to the gills, where it exchanges
carbonic acid for oxygen ; from the gills by efferent branchial
veins to the branchio-cardiac veins, thence into the peri-
cardial sinus, and so to the heart once more.
It will be seen that the circulatory system of the crayfish
consists of three sections — (i) the heart or organ of pro-
pulsion ; (2) a system of out-going channels, the arteries,
which carry the blood from the heart to the body generally ;
and (3) a system of returning channels — some of them, the
sinuses, mere irregular cavities, others, the veins, with
definite walls — these return the blood from the various
organs back to the heart. The respiratory organs, it should
be observed, are interposed in the returning current, so that
blood is taken both to and from the gills by veins.
340
THE CRAYFISH
LESS.
Comparing the blood-vessels of Astacus with those of
Polygordius, it would seem that the ophthalmic artery,
heart, and dorsal abdominal artery together answer to the
dorsal vessel, part of which has become enlarged and mus-
cular, and discharges the wlioie function of proi)elling the
hi
Fic. 88. — Diagram illustrating the course of the circulation of the blood
in the Crayfish.
Heart and arteries ie<l ; veins and sinuses containing non-aerated
blood blue : veins and sinuses containing aerated blood pink.
The arrows show the direction of the flow.
Tlie blood from the p<*ricardial sinus {fed. s) enters the heart {/d) by
a valvular aperture {v^) and is propelled into arlcries («), the orifices of
which are guarded by valves { 7 /^) : the ultimate branches of the arteries
discharge the blood into sinuses (j), an<l the sinuses in various parts of
the body debouch into the sternal sinus (j/. s) : thence the blood is taken
by the aflerent branchial veins {a/. l>r. v) into the gilb, where it is purified
and is returned by elTerent branchial veins {ef. br. v) into the branchio-
cardiac veins {br. c. z') which open into the pericardial sinus.
(Fn)m Parker and Haswell's Zoolo^.^
blood. The horizontal portion of the sternal artery, together
with the ventral abdominal, represent the ventral vessel,
while the vertical portion of the sternal artery is a com-
missure, developed sometimes on the right, sometimes on
the left side, its fellow being suppressed.
xxvn
NERVOUS SYSTEM
341
The blood when first drawn is colourless, hut after ex-
posure to the air takes on a bluish-gray tint. This is owing
to the presence of a colouring matter called Juvmocyanin^
which becomes blue when combined with oxygen ; it is a
respiratory pigment, and serves, like haemoglobin, as a
carrier of oxygen from the external medium to the tissues.
The hxmocyanin is contained in the plasma of the blood :
the corpuscles are all leucocytes.
The nervous system consists, like that of Polygordius, of
a brain (Fig. 86, g) and a ventral nerve-cord (hn\ united by
cesophageal connectives. But the ventral nerve-cord is
differentiated into a series of paired swellings or gx7tglia to
which the nerve-cells are confined, united by longitudinal
connectives. The brain supplies not only the eyes and
antennules, but the antennae as well, and it is found by
development that the two pairs of ganglia belonging to the
antennulary and antennary segments have fused with the
brain proper. Hence we have to distinguish between a
primary brain or archi-cerebrum^ the ganglion of the prosto-
mium, and a secondary brain or syn-cerebrum formed by the
union of one or more pairs of ganglia of the ventral cord
with the archi-cerebrum. A further case of concrescence of
ganglia is seen in the ventral nerve-cord, where the ganglia
of the last three cephalic and first three thoracic segments
have united to * form a large compound sub-oesophageal
ganglion. All the remaining segments have their own
ganglia, with the exception of the telson, which is supplied
from the ganglion of the preceding segment. There is a
visceral system of nerves supplying the stomach, originating
in part from the brain and in part from the oesophageal
connectives.
The eyes have a very complex structure. The chitinous
cuticle covering the distal end of the eye-stalk is transparent,
342
THE CRAYFISH
LESS.
divided by delicate lines into square areas or facets^ and
constitutes the cornea. Beneath each facet of the cornea is
an a|)f)aratus called an ommafidcinn, consisting of an outer
segment or viireous My having a refractive function, and
ail inner segment or retinula forming the actual visual
portion of the apparatus. 'Fhe ommatidia are optically
separated from one another by black pigment, so that each
is a distinct organ of sight, and the entire eye is called a
compound eye.
'I’he antennules contain two sensory organs, to which are
usually assigned the? functions of smell and hearing respe*'*
lively. 'Vhe o//(/c/ory'* orji^^an is constituted by a luunber
of delicate olfactory seta\ borne on the external flagellum and
supplied by the antennulary nerve. 1'he “ auditory ” ori^^an
or statocyst is a sac formed by invagination of the dorsal
surface of the proximal segment, and is in free communi-
('ation with the surrounding water by a small a[)erture. I'he
Glutinous lining of the .sac is produced into delicate feathered
auditory sette^ .sui)i)lied by branches of the antennulary
nerve, and in the water which fdls the sac are minute sand-
grains, which take the place of the otoliths or ear-stones
found in most auditory organs, but which, instead of being
formed by the animal itself, are taken in after each eedysis,
when the lining of the sac is shed. Many of the setai on
the general surface of the body have a definite nerve-supply,
and are probably tactile organs.
The crayfish is dioecious, and presents a very obvious
sexual dimorphism or structural difference between male
and female, apart from the actual organs of reproduction.
'Fhe abdomen of the female is much broader than that of
the male : the first and second pleo[)ods of the male are
modified itito tubular or rather spout-like copulatory organs ;
and the reproductive aperture is situated in the male on the
XXVII REPRODUCTIVE ORCiANS 343
proximal podomere of the fifth leg, in the female on that of
the third.
The spennary {V\g, 86, t) lies in the thorax, just beneath the
lloor of the pericardial sinus, and consists of paired anterior
lobes and an impaired posterior lobe. From each side goes
off a convoluted spermiduct or vas deferens (yd\ which opens
on the proximal segment of the last leg. The sfierms are
curious non-motile bodies produced into a number of stiff
processes : they are aggregated into vermicelli-like sper-
tnaiophores by a secretion of the vas deferens.
The ovary is also a three-lobed body, and is similarly
situated to the testis : from each side proceeds a thin-walled
oviduct^ which passes downwards, without convolutions, to
open on the proximal segment of the third or antepenulti-
mate leg. The eggs are of considerable size and contain a
great cpiantity of yolk {see p. 256).
Both ovary and testis are hollow organs, discharging their
products internally. I'heir cavities represent the ccelome,
and their ducts are organs of the same general nature as
nephridia. The ova, when laid, are fastened to the setie on
the pleopods of the female by the sticky secretion of glands
occurring both on those appendages and on the segments
themselves : they are fertilised immediately after laying, the
male depositing spermatophores on the ventral surface of
the female’s body just before oviposition.
The process of segmentation of the oosperm presents
certain striking peculiarities. The nucleus divides repeatedly
(Fig. 89, A, nu\ but no corresponding division of the pro-
toplasm takes place, with the result that the morula-stage,
instead of being a heap of cells, is simply a multinucleate
but non-cellular body. Soon the nuclei thus formed retreat
from the centre of the embryo, and arrange themselves in a
single layer close to the surface (b) : around each of these
344
THI-: CRAYFISH
LESS.
proto[)]asni ;« ( (i]niilat<-s, the central part of the embryo
(onsistin |4 entirely ot yolk iiiiileiial. thus get a super-
Jldal .sr^nu'}il(Uio7i^ ( hara< lerised by a central mass of yolk
and a Mipeiiicial layer of n-ll.s collectively known as the
l>Iast()derf?i (c).
On one j)()le an invagination of the blastoderm takes
place, giving iis( U) a small sac, the enteron, which cotnmii-
nic'att's with the (wU iidi by an apeiliiu', vhe blastopore. By
tliis [)roeess the ('ni])ryo [)ass('s into the gastriila-stagc, which,
however, differs from the corresponding stage in Polygordius
Flea 89. — Three sl.i^cs in tlu‘ e.nh (l<'\(T>nnicnl ot the ('r.uhsh.
Ill A llic' pmducK of cIiMMon o| ihc iiurltais (////I .im su n in tlir
I’ciiiK' ()l the oilk ; ill ic and c liu’ nni'lci !u\(‘ anaiii^cd ilu'iiisr!\(u in a
pcriphernl layer, each surrounded l>y piotoplasni, so as lo fuini die
IdaslodLTin.
(Fmni Parker and Haswell’s Zodogy^ after Morin.)
(p. .icjs) in the innneiise (|iiantity of food-yolk fdling up
the spacA' (blastocac-le) heh\een ec toderin and endoderni.
Wav soon tlu' embryo luronies ti iploblastie, or tlm c !.i\i od,
by the' budding olf of c'ells bom tin* taidodcnn in thi' neigh-
bourhood ol the' blastopore : these ac camudati between
the eetodcain and c ndodcam, and c'onstuutc' the mesocUam.
Bedore long the blastopore' closes, and a slomocLeiim and
proc’todaum (p. 2(p) arc lornied as invagdnations (d' the
cc'loderin uhic'h t'\entuall\ eonmuinicate with the enteron,
forming a eoiii[)iete enteric canal. On each side of the mouth
XX VI I
DEVELOPMENT
345
or aperture of ilu' stomodival depression ( I'Il;. no) thns' (‘leva-
tions a{)p(‘ar, tiu' riidinients (T tlu' aiitenniili s (^/’), antenna*
and niandihks (///) : in front of them is anotlu i pair
of elevations on which the eyes (. /) siihsecjuently appear.
p’lO. 90. — Early em])ryo of I‘’iesh-wa1cr Crayfish in the nauplius
stage.
A inthcupjx r j^ni! of tin* fiL'mc i'.llif ('\f : /, tin- labnun o\ ciliaiiging
the mouth, (»n t\!< h ol whith ;iic tin ni'inm iit ^ of iIm- aiitfuniilc s
(</h» antt'iiii <' sial ni.uiaihh -> (;//) * t.< huu) Uk hi i\ ihc iiitliitK ni of
the thorax aii'l alxloii}' 11 {/ /)\\ith the amis (.'/ ) '1 he nn ijmcnl , «>/
the first thu-t' j):um <»( iMir.hi.i (O', ^e//) an* seen lliiuiiL'.li the lians
parent ectoderm.
(From Lang, after la iv h* iilodi,)
An unpaired Lli vation (T.4) behind the mouth, and having
the anus (. /) or ajierture of the j)roetodaMl depiession at its
summit, is the rudiment of the thorax and abdomen, d'he
embryo is now called a nauplius. Many Crustacea are
346
THK ('RAVMSll
LKSS.
hatched in the form of a fire swi nun ini; l;n\a, lo whuh thi.s
name is a])[)li{‘d, ('hara<'ti risrd 1>\ tin* presence of three
pairs of appendages, used for swimming and becoming the
f'lc;. 91. — Later em])ryo of Fresh-water Craytish, from the ventral
.is[)ect ; the abdomen (a/d is folded down over the cephalothorax, so
that its dorsal surface faces the observer, and the telson (T) reaehes
nearly to the mouth.
The followinii:^ appendages are indicated : A, eye-stalks ; ab anten-
nules ; <i~, antenn.v ; m, mandibles; ///.v’, wa-, maxilhe ; t. I-/. 8,
ihorauc appendages (maxillipiMles and legs).
At the sides of the thoiax aie seen the etlges of the carapace (/.v) : in
fioiU of the mouth is the labriim (/), in front of the labriim the brain (.^d>
and at tlie base of tlie eye stalk the oi)tic ganglion
(Funn Lang, after Reichenb.u h. )
antcnniilcs, antcnntc, and mandibles of adult. In the
crayfish there' is no free* larva, and the' nauplius stage is
passed through iH'fori' hatching.
d’he naupluis is gradually transformed into the crayfish by
XXVII
DEVELOPMENT
347
the appearance of fresh appendages, in regular order, behind
the first three (Fig. 91) ; by the elongation of the rudiment
of thorax and abdomen (a/f) ; and by the gradual differen-
tiation of the ajjpendages. When hatched the young
animal agrees in all essential respects with the adult, but its
proportions are very different, the cephalothorax being nearly
globular and the abdomen small, h^or some lime after
hatching the young crayfishes cling in great numbers to the
pleopods of the mother by means of the peculiarly hooked
chelic of the first pair of legs.
LESSON XXVIII
THE FRESH-WATER MUSSEL
In the mussel we meet with an entirely new type of
structure : the animal is bilaterally symmetrical, with no
trace of metameric segmentation ; the power of locomotion
is greatly restricted, and food is obtained passively by ciliary
action, as in Infusoria, not by the active movements of
definite seizing organs — tentacles, limbs, or protrusible
mouth — as in most of the higher animal forms.
Fresh-water mussels are found in rivers and lakes in most
parts of the world, Anodonta cygnea, the swan-mussel, is
the commonest species in England ; but the pearl-mussel,
Unio margarififer^ is found in .mountain streams, and other
species of the same genus are universally distributed.
The mussel is enclosed in a brown shell formed bf two
separate halves or valves hinged together along one edge.
It lies on the bottom, partly buried in the mud or sand,
with the valves slightly gaping, and in the narrow cleft thus
formed a delicate, semi-transparent substance is seen, the
edge of the rmmtk or palliitm. The mantle really consists
of separate halves or lobes corresponding with the valves of.
LESS. XXVIII
GENERAL STRUCTURE
349
the shell, but in the position of rest the two lobes are so
closely approximated as to appear simply like a membrane
uniting the valves. At one end, however, the mantle pro-
jects between the valves in the form of two short tubes, one
(Fig. 92, B, ex. smooth-walled, the other (/W. s/>/i.) lieset
with delicate processes or fimbruc. By diffusing particles of
carmine or indigo in the water it can be seen that a current
is always passing in at the fimbriated tube, hence called the
inhalant siphon^ and out at the smooth or exhalant siphon.
Frequently a semi-transparent, tongue-like body (y?) is pro-
truded between the valves at the opposite side from the
hinge and at the end furthest from the siphons : this is the
footy by its means the animal is able slowly to plough its
way through the sand or mud. When irritated the foot and
siphons are withdrawn and the valves tightly closed. In a
dead animal, on the other hand, the shell always gapes, and
it can then be seen that each valve is lined by the corre-
sponding lobe of the mantle, that the exhalant siphon is
formed by the union of the lobes above and below it and
is thus an actual tube, but that the boundary of the inhalant
siphon facing the gape of the shell is simply formed by the
approximation of the mantle-lobes, so that this tube is a
temporary one.
The hinge of the shell is dorsal, the gape ventral, the end
bearing the siphons posterior, the end from which the foot
is protruded anterior : hence the valves and mantle-lobes
are respectively right and left.
In a dead and gaping mussel the general disposition of
the parts of the animal is readily seen. The main part of
the body lies between the dorsal ends of the valves : it is
produced in the middle ventral line into the keel-like foot :
and on each side, between the foot and the corresponding
mantle-lobe, are two delicate, striated plates, the gills. Thus
350
THE FRESH-WATER MUSSEL
LESS.
the whole animal has been compared to a book, the back
being represented by the hinge, the covers by the valves,
the fly-leaves by the mantle-lobes, the two first and the two
last pages by the gills, and the remainder of the leaves by
the foot.
When the body of the mussel is removed from the shell
the two valves are seen to be united, along a straight hinge-
line (Fig. 92, a, h. /), by a tough, elastic substance, the
hinge-ligament (Fig. 93, B, lig) passing transversely from valve
to valve. It is by the elasticity of this ligament that the
shell is opened : it is closed, as we shall see, by muscular
action : hence the mere relaxation of the muscles opens the
shell. In Anodonta the only junction between the two
valves is aflbrded by the ligament, but in Unio each is pro-
duced into strong projections and ridges, the hinge-teeth^
separated by grooves or sockets, and so arranged that the
teeth of one valve fit into the sockets of the other.
The valves are marked externally by a series of concentric
lines parallel with the free edge or ga})e, and starting from
a swollen knob or elevation, the umbo^ situated towards
the anterior end of the hinge-line. These lines are lines of
gnnvth. The shell is thickest at the umbo, which represents
the part originally formed, and new layers are deposited
under this original portion, as secretions from the mantle,
the shell being, like the armour of the crayfish, a cuticular
exoskeleton. As the animal grows each layer projects
beyond its predecessor, and in this way successive outcrops
are produced giving rise to the markings in question.
In the region of the umbo the shell i^ ij^ually more
or less eroded by the action of the carbonic acid in the
water.
The inner surface of the shell also presents characteristic
markings (Fig. 92, a). Parallel with the gape, and at a
Fui. 92. — A, interior of right valve of Anodonta, showing the various
impressions produced by the muscles shown in B : h. /, hinge-line ; pL 1 .
pal Hal line.
B, the animal removed from the shell and seen from the left side.
a. ady anterior adductor ; a. r, anterior retractor ; d. .i,'-, digestive gland,
seen through mantle ; ex. sph, cxhalant siphon ; hjot ; gills, seen
through mantle ; in. sphy inhalant siphon ; kdy kidney, seen through
mantle ; k. Oy Keljer’s organ, seen through mantle ; niy mantle ; p. ady
posterior adductor ; pCy pericardium, seen through mantle ; pi. niy pallial
muscles ; p. r, j;osterior retractor ; prcy protractor.
(From Parker and IlaswelPs Zoology.)
short distance from it, is a delicate streak (//. I) caused by
the insertion into the shell of muscular fibres from the edge
of the mantle : the streak is hence called the pallial line.
XXV III
352
THE FRESH WATER MUSSEL less, xxviii
Beneath the anterior end of the hinge the pallial line ends
in an oval mark, the afiterior adductor impression {a, ad\
into which is inserted one of the muscles which close the
shell. A similar, but larger, posterior adductor impression
(/. ad) lies beneath the posterior end of the hinge. Two
smaller markings in close relation with the anterior adductor
impression mark the origin of the anterior retractor (a. r\
and of the protractor {prc) of the foot : one connected with
the posterior adductor impression, that of the posterior
retractor (/. r) muscle. P'rom all these impressions
faint converging lines can be traced to the umbo: they
mark the gradual shifting of the muscles during the growth
of the animal.
The shell consists of three layers. Outside is a brown
horn-like layer, the periostracum, composed of conchioUn^ a
substance. allied in composition to chitin. Beneath this is a
prismatic layer formed of minute prisms of calcium carbon-
ate, separated by thin layers of conchiolin; and, lastly,
forming the internal part of the shell is the nacre^ or
“ mother-of-pearl,” formed of alternate layers of carbonate of
lime and conchiolin arranged parallel to the surface. The
periostracum and the prismatic layer are secreted from the
edge of the mantle only, the pearly layer from the whole of
its outer surface. The hinge-ligament is continuous with
the periostracum, and is to be looked upon simply as a
median uncalcified portion of the shell, which is therefore,
in strictness, a single continuous structure.
By the removal of the shell the body of the animal
(Fig. 92, b) is seen to be elongated from before backwards,
narrow from side to side, produced on each side into a
mantle-lobe (w), and continued ventrally into a keel-like
visceral mass^ which passes below and in front into the
foot {ft\ Thus each valve of the shell is in contact with
Ctel .JFpfhm
''CctlJC^thmr
^Itrnt’
Fig. 93 . — Diagrammatic sections of the Fresh-water Mussel.
A, longitudinal section*: right mantle-lobe {Afant) and gills (/. Gy
O. G) are shown in perspective.
B, transverse section.
The cuticular shell {Sk)y shown only in B, is black, the ectoderm
dotted, the nervous system finely dotted, the endoderm radially striated,
the mesoderm evenly shaded, and the ccelomic epithelium represented
by a beaded line.
The dorsal region is produced into the right and left manlle-lobcs
{Afan^)j attached to which are the valves of the shell (67/) joined dorsally
by an elastic ligament (/^^).
The mantle-lobes are partly united so as to form the inhalant (/«//.
A/>) and exhalant {£xk. Ap) apertures at the posterior end.
The body is produced ventrally into the foot i^Foot^y on each side of
which are the gills, an inner (/. G) and an outer {O. G)y each formed
of an inner and an outer lamella.
The body is covered externally by deric epithelium {Der. Epthm)^
within which is mesoderm {Msd) largely differentiated into muscles, of
which the anterior {A. Ad) and posterior {P. Ad) adductors are indi-
cated in A.
The mouth {Mth) leads by the short gullet {Gul) into the stomach
{St)y from which proceeds the coiled intestine {/nt), ending in the anus
A A
354
THE FRESII-WATER MUSSEL
I.ESS.
{An ) : the enteric epithelium is mostly endodermal. The digestive gland
{D. Gl) surrounds the stomach. The ccelome (CVr/) is reduced to a
small dorsal chamber enclosing part of the intestine and the heart ; the
parietal {Ca’l, Epthm) and visceral {Cccl. Epthm^) layers of coelomic
epithelium are shown.
The heart consists of a median ventricle ( Vent\ enclosing part of the
intestine, and of paired auricles {Aur).
The paired nephridia {Nphni) open by apertures into the coelome
{Nph, St) and on the exterior {Np/f. p).
The gonads {Gon) are imbcddcfl in the solid mesoderm, and open on
the exterior by gonoducts (Gn^f).
The nervous system consists of a pair of cercbro- pleural ganglia
{C. P, above the gullet, a pair of pedal ganglia (/V/. Gn) in the
foot, and a pair of visceral ganglia ( F. Gn) below the posterior adductor
muscle.
the dorso-lateral region of the body of its own side, together
with the corresponding mantle-lobe, and it is from the epi-
thelium (Fig. 93, Der. Epthm) covering these parts that the
shell is formed as a cuticular secretion. The whole space
between the two mantle-lobes, containing the gills, visceral
mass, and foot is called the mantle-cavity,
A single layer of epithelial cells, the deric epithelium or
epidermis {Der, Epthm\ covers the whole external surface,
i,e,y the body proper, both surfaces of the mantle, the gills,
and foot; that of the gills and the inner surface of the
mantle is ciliated. Beneath the epidermis come connective
and muscular tissue, which occupy nearly the whole of the
interior of the body not taken up by the viscera, the coelome
being, as we shall see, much reduced. The muscles are all
unstriped, and are arranged in distinct bands or sheets,
many of them very large and conspicuous. The largest are
the anterior and posterior adductors (P'igs. 92, 93^ and 94,
a, ad^ p, ad)y great cylindrical muscles which pass trans-
versely across the body and are inserted at either end into
the valves of the shell, which are approximated by their
contraction. Two muscles of much smaller size pass from
the foot to the shell, which they serve to draw back : they
XXVIII
DIGESTIVE ORGANS
55S
are called the anterior (a. r) and posterior {p, r) retractors.
A third muscle (prc) is inserted into the shell close
to the anterior adductor, and has its fibres spread fan-wise
over the visceral mass which it serves to compress, thus
forcing out the foot and acting as a protractor of that organ.
The substance of the foot itself consists of a complex mass
of fibres, the intrinsic muscles of the foot, many of which
also act as protractors. Lastly, all along the border of the
mantle is a row of delicate pallial muscles (Fig. 92, b, //.
m), which, by their insertion into the shell, give rise to the
pallial line already seen.
The coelome is reduced to a single ovoidal chamber, the
pericardium (Fig. 93, Coel ; Fig. 94, /r), lying in the dorsal
region of the body and containing the heart and part of the
intestine : it is lined by coelomic epithelium ( Ccel. Epthm\
and does not correspond with the pericardial sinus of the
crayfish, which is a blood-space. In the remainder of the
body the space between the ectoderm and the viscera is
filled by the muscles and connective tissue.
The mouth (Fig. 94, mtli) lies in the middle line, just
below the anterior adductor. On each side of it are two
triangular flaps, the internal and external labial palps; the
external palps unite with one another in front of the mouth,
forming an upper lip; the internal are similarly united
behind the mouth, forming a lower lip : both are ciliated
externally. The mouth leads by a short gullet (Fig. 94,
gut) into a large stomach {st\ which receives the ducts of a
pair of irregular, dark-brown digestive glands (d.gt). The
intestine (int) goes off from the posterior end of the stomach,
descends into the visceral mass, where it is coiled upon
itself, then ascends parallel to its first portion, turns sharply
backwards, and proceeds, as the rectum {rct\ through the
pericardium, where it traverses the ventricle of the heart,
A A 2
mm
Fig. 94 . -Dissection of Anodonta, made by removing the mantle-
lobe, inner and outer gills, wall of pericardium, and auricle of the left
side, and dissecting away the skin, muscles, &c. of the same side down
to the level of the enteric canal, kidney, nervous system, &c. Part of
the enteric canal is laid open, as also are the kidney {ki} and bladder
[1)1], The connection between the cerebro-pleural [c, € ^] and
visceral [v. gn) ganglia is indicated by a dotted line.
a, anus ; a. ad, anterior adductor ; a. ao, anterior aorta ; a, v. ap,
auriculo-ventricular aperture ; bl, urinary bladder ; f. pi gn, cerebro-
pleural ganglion : d. d, duat of digestive gland ; d. digestive gland ;
d.p. a, dorsal pallial aperture ; cx. spk, exklant siphon ; //, foot ; g, ap,
genital aperture ; gon, gonad ; gullet ; /. I inter-lamellar junc-
tion ; in, spk, inhalant siphon ; int, intestine ; kd, kidney ; m, mantle ;
mtk, mouth ; p. ao, posterior aorta; p. ad, posterior adductor ; pc, peri-
cardium ; pd. gn, pedal ganglion ; r. ap, renal aperture ; r, au, right
auricle ; ret, rectum ; r, p. a, reno-pericardial aperture ; st, stomach ;
iy, typhlosole; v, ventricle; v. gn, visceral ganglion; w, t, water-
tubes ; X, aperture between right and left urinary bladders.
(From Parker and Haswell’s Zoolo^.]
and above the posterior adductor, finally discharging by the
anus {a) into the exhalant siphon, or cloaca. The wall of
xxvm
GILLS
357
the rectum is produced into a longitudinal ridge, or typhlosole
(fy)y and two similar ridges begin in the stomach and are
continued into the first portion of the intestine. The
stomach contains at certain seasons of the year a gelatinous
rod, the crystalline style.
The gills consist, as we have seen, of two plate-like bodies
on each side between the visceral mass and the mantle : we
have thus a right and a left outer (Fig. 93, B, O. G\ and a
right and a left inner gill (/. G). Seen from the surface
(P’ig. 94), each gill presents a delicate double striation,
being marked by faint lines running parallel with, and by
more pronounced lines running at right angles to, the long
axis of the organ. Moreover, each gill is double, being
formed of two similar plates, the inner and outer lamellcBy
united with one another along the anterior, ventral, and
posterior edges of the gill, but free dorsally. The gill has
thus the form of a long and extremely shallow bag open above
(Figs. 94 and 95) : its cavity is subdivided by vertical plates
of tissue, the inter-lamellar junctions (Fig. 95, i. /. y), which
extend between the two lamellae and divide the intervening
space into distinct compartments or water-tubes {w. t),
closed ventrally, but freely open along the dorsal edge of
the gill. The vertical striation of the gill is due to the fact
that each lamella is made up of a number of close-set gill-
filaments (/) : the longitudinal striation to the circumstance
that these filaments are connected by horizontal bars, the
inter-filamentar junctions (/. f j). At the thin free or ventral
edge of the gill the filaments of the two lamellae are con-
tinuous with one another, so that each gill has actually a
single set of V-shaped filaments, the outer limbs of which
go to form the outer lamella, their inner limbs the inner
lamella. Between the filaments, and bounded above and
below by the inter-filamentar junctions are minute apertures,
.358
TIIK I'kl'SH.WAl'KK MI SSKL
l Kss,
or ostia {os)^ wliich lead from the mantle-cavity through a
more or less irregular series of cavities into the interior of
the water tubes. The filaments themselves are supported
by chitinous rods, and covered with ciliated epithelium, the
large cilia of which produce a current running from the
exterior through the ostia into the water-tubes, and finally
Fin. 95. — Diag;ram of the structure of the gill of Anodonla.
The gill is made up of V-shaped gill-filaments {/) arranged in longi-
tudinal seiies and bound together by horizontal inter-filamenlar junctions
(/. f. j) which cross them at right angles, forming a kind of basket-work
with apertures, the ostia (<7.9), leading from the outside and opening {os')
into the cavity of the gill. The latter is divided by vertical partitions,
the inter-lamellar junctions {^'. /. /), into compartments or water-lubes
{7V. /) which open also into the supra-branchial chamber ; d. v, blood-
vessels.
(From Parker and Has well’s Zoology.)
escaping by the wide dorsal apertures of the latter. The
whole organ is traversed by blood-vessels [b. v).
The mode of attachment of the gills presents certain
features of importance. The outer lamella of the outer gill
is attached along its whole length to the mantle : the inner
XXVIII
EXCRETORY ORGANS
359
lamella of the outer, and the outer lamella of the inner gill
are attached together to the sides of the visceral mass a
little below the origin of the mantle : the inner lamella of
the inner gill is also attached to the visceral mass in front,
but is free further back. The gills are longer than the
visceral mass, and project behind it, below the posterior
adductor (Fig. 94), as far as the posterior edge of the
mantle : in this region the inner lamellcC of the inner gills
are united with one another, and the dorsal edges of all
four gills constitute a horizontal partition between the pallial
cavity below and the exhalant chamber or cloaca above.
Owing to this arrangement it will be seen 'that the water-
tubes all open dorsally into a supra-branchial chamber^ con-
tinuous posteriorly with the cloaca and thus opening on
the exterior by the exhalant siphon.
The physiological importance of the gills will now be
obvious. By the action of their cilia a current is produced
which sets in through the inhalant siphon into the pallial
cavity, through the ostia into the water-tubes, thence into the
supra-branchial chamber, and out at the exhalant siphon.
The in-going current carries with it not only oxygen for the
aeration of the blood, but also diatoms, infusoria, and other
microscopic organisms, which are swept into the mouth by
the cilia covering the labial palps. The out-going current
carries with it the various products of excretion and the
faeces passed into the cloaca. The action of the gills in
producing the food-current is of more importance than their
respiratory function, which they share with the mantle.
The excretory organs are a single pair of curiously-modified
nephridia^ situated one on each side of the body just below
the pericardium. Each nephridium consists of two parts, a
brown spongy glandular portion or kidney (Fig. 94, kd)y and
a thin-walled non-glandular part or bladder (pi). The two
36 o the fresh-water MUSSEL less.
parts lie parallel to one another, the bladder being placed
dorsally and immediately below the floor of the pericardium:
they communicate with one another posteriorly, while in
front the kidney opens into the pericardium (r, f, ap\ and
the bladder on the exterior by a minute aperture (r. ap\
situated between the inner gill and the visceral mass. Thus
the whole organ (Fig. 93, Nphm\ often called after its dis-
coverer, the or^an of Bojanus, is simply a tube bent upon
itself, opening at one end into the coelome {Nph, st\ and at
the other on the external surface of the body {Nph. p) : it
has therefore the normal relations of a nephridium. The
epithelium of the bladder is ciliated, and produces an
outward current.
It seems probable that an excretory function is also dis-
charged by a large glandular mass of reddish-brown colour,
called the pericardial gland or Keber's organ (Fig. 92, B,
k, o). It lies in the anterior region of the body just in front
of the pericardium, into whicA it discharges.
The circulatory system is well developed. The heart lies
in the pericardium, and consists of a single ventricle (Fig. 93,
Vent^ and Figs. 94 and 96, v) and of right and left auricles
{au). The ventricle is a muscular chamber which has the
peculiarity of surrounding the rectum (Figs. 93 and 94) :
the auricles are thin-walled chambers communicating with
the ventricle by valvular apertures opening towards the
latter. From each end of the ventricle an artery is given
off, the anterior aorta (Fig. 94, a. ao) passing above, the
posterior aorta {p. ao) below the rectum. From the aortae
the blood passes into arteries (Fig. 96, art.^^ art^) which
ramify all over the body, finally forming an extensive net-
work of vessels, many of which are devoid of proper walls
and have therefore the nature of sinuses. The returning
blood passes into a large longitudinal vein, the vena cava
XXVIII
CIRCULATORY SYSTEM
361
{v. c)f placed between the nephridia, whence it is taken to
the kidneys themselves (ttpL v\ thence by aff€re7tt branchial
Fig. 96. — Diagram of the Circulatory System of Anodonta.
The blood received from the auricles {an) is pumped by the ventricle
(z/) into the aorta {ao) and thence passes to the mantle {art.^) and to
the body generally {art.'^).
The blood which has circulated through the mantle is returned
directly to the auricle : that from the body generally is collected into
the vena cava {v. r), passes by nephridial veins {nph) to the kidneys,
thence by afferent branchial veins {af. br. v) to the gills, and is returned
by efferent branchial veins {ef. br. v) to the auricles ; pc, pericardium.
(From Parker and HaswelPs Zoology.
veins (af. br. v) to the gills, and is finally returned by efferent
branchial veins (ef. br. v) to the auricles. The mantle has a
very extensive blood -supply, and probably acts as the chief
362
THE FRESH-WATER MUSSEL
LESS.
respiratory organ : its blood (ar/^) is returned directly to the
auricles without passing through either the kidneys or the
gills. The blood is colourless and contains leucocytes.
There is no communication between the blood-system and
the pericardium.
The nervous system is formed on a type quite different
from anything we have yet met with. On each side of the
gullet is a small cerebro-pleural ganglion (Fig. 94, c, pL g/i\
united with its fellow of the opposite side by a nerve-cord,
the cerebral comm/ssi/re, passing above the gullet. Each
ccrebro-pleural ganglion also gives off a c'ord, the cerebro-
pedal connective^ which passes downwards and backwards to
a pedal ganglion {pd. gn) situated at the junction of the
visceral mass with the foot : the two pedal ganglia arc so
closely united as to form a single bilobed mass. From each
cerebro-pleural ganglion there further proceeds a long cerebro-
visceral connective^ which passes directly backwards through
the kidney, and ends in a visceral ganglion (v. gn) placed oh
the ventral side of the posterior adductor muscle. The
visceral, like the pedal ganglia, are fused together. The
cerebro-pleural ganglia supply the labial palps and the
anterior part of the mantle ; the pedal, the foot and its
muscles ; the visceral, the enteric canal, heart, gills, and
posterior portion of the mantle.
It will be seen that the cerebral commissures and cerebro-
pedal connectives, together with the cerebro-pleural and
pedal ganglia, form a nerve-ring which surrounds the gullet :
the cerebro-pleural ganglia may be looked upon as a supra-
oesophageal nerve mass corresponding with the brain
of Folygordius and the Crayfish, and the pedal ganglia
as an infra-oesophageal mass representing the ventral nerve
cord.
Sensory organs are poorly developed, as might be ex-
XXVIII
DEVELOPMENT
363
pected in an animal of such sedentary habits. In connec-
tion with each visceral ganglion is a patch of sensory
epithelium forming the so-called olfactory organ or, better,
osphradium^ the function of which is apparently to test the
purity of the water entering by the respiratory current.
Close to the pedal ganglion is a minute otocyst or staiocyst^
the nerve of which is said to spring from the cerebro-pedal
connective, being probably derived from the cerebral gang-
lion. Sensory cells, probably tactile, also occur round the
edge of the mantle, and especially on the fimbria; of the in-
halant siphon.
The sexes are separate. The gonads (Figs. 93 and 94,
goti) are large, paired, racemose bodies, occupying a con-
siderable portion of the visceral mass amongst the coils of
the intestine : the spermary is white, the ovary reddish. The
gonad of each side has a short duct which opens (^. ap) on
the surface of the visceral mass, just in front of the renal
aperture.
In the breeding season the eggs, extruded from the genital
aperture, pass into the supra-branchial chamber, and so to
the cloaca. There, in all probability, they are impregnated
by sperms introduced with the respiratory current. The
oosperms are then passed into the cavities of the outer gills,
which they distend enormously. Thus the outer gills act as
brood-pouches, and in them the embryo develops into the
peculiar larval form presently to be described.
The segmentation of the oosperm is remarkable for the
fact that the cells of the polyplast are of two sizes, small
cells composed entirely of protoplasm, and large cells loaded
with yolk -granules. In the formation of the gastrula the
large are invaginated into the small cells, but the enteron
thus formed is very small and quite unimportant during
early larval life, the young mussels being nourished, after
364
THE FRESII-WATER MUSSEL
the manner of parasites, by a secretion from the gills of the
parent.
1’he dorsal surface of the embryo is soon marked out by
the appearance of a deep depression, the shell-^land^ which
secretes, in the first place, a single median shell. This is,
however, soon replaced by a bivalved larval shell (Fig. 97,
j), of triangular form, the ventral angles being produced into
hooks (sH), The body at the same time becomes cleft from
Fig. 97. — A, advanced embryo of Anodonta enclosed in the egg-mem-
brane. B, free larva or glochidium.
/, byssus ; g, lateral pits ; j, shell ; hooks ; Jw, adductor muscle ;
sensory hairs ; 7«/, ciliated area.
(From Korschelt and Heider.)
below upwards (a), forming the right and left mantle-lobes.
On the ventral surface, between the lobes of the mantle,
is formed a glandular pouch, which secretes a bunch of
silky threads, the byssus (/). The larva is now called a
glochidium.
The glochidia, entangled together by means of their
byssal threads, escape from the gills of the parent by the
xxvin
METAMORPHOSIS
365
exhalant siphon, and eventually attach themselves, by their
hooked valves, to the body of a passing fish, such as a
stirklel)ack. Here they live for a time as external parasites,
gradually undergoing metamorphosis ; and finally dro]) from
the host and assume the sedentary habits of the adult.
LESSON XXIX
THE DOGFISH
The animals studied in the three previous Lessons have
served to illustrate three widely different types of organiza-
tion. The starfish is radially symmetrical, with an under-
lying bilateral symmetry, and no indication of metamerism :
the crayfish is bilaterally symmetrical, metamerically seg-
mented, and provided with numerous limbs, both trunk and
limbs being covered with a hard, jointed armour or exo-
skeleton ; the mussel is likewise bilaterally symmetrical,
covered with a shell formed of paired pieces, and having no
indication of metamerism, and no trace of limbs. We have
now to consider, in the dogfish, an animal belonging to the
great group of Vertebrata, in which the bilaterally symme-
trical body is definitely divided into metameres, although
there is no indication of the fact externally. There are only
two pairs of limbs or paired appendages, and the main sup-
porting structures are a complicated internal system of
articulated hard parts, forming the endoskeieton or internal
skeleton.
The commonest British dogfishes are the Rough Hound
368
THE DOGFISH
LESS.
{Scyllium canicula\ the Lesser Spotted Dogfish ( 5 . catulus\
the Piked Dogfish {Acanthias vulgaris\ and the Smooth
Hound {Mu steins vulgaris). The following description,
though referring mainly to Scy Ilium., will apply, in essential
respects, to any of these.
1 "he dogfish has a spindle-shaped body (Fig. 98), ending
in front in a bluntly-pointed snout or cut-water, and behind
tapering off into an upturned tail. On the ventral surface
of the head is the large, transversely elongated mouth (mth),
supported by a pair of jaws which work in a vertical, and
not, like those of the crayfish, in a transverse plane, and
arc, in fact, portions of the skull, having nothing to do with
limbs. They are covered with teeth which vary in form in
the different species. In front of the mouth, on the ventral
surface of the snout, are the paired nostrils {na\ each lead-
ing into a cup-like nasal sac. The eyes {e) are also two in
number and are placed one on each side of the head, above
the mouth. Behind the mouth are five slit-like apertures
{ex. br. ap\ arranged in a longitudinal series : these are the
gill-clefts or external branchial apertures. Just behind the
eye is a small aperture, the spiracle {sp) : like the gill-clefts,
it communicates with the pharynx, and it is found by de-
velopment to be actually the functionless first gill-cleft.
On the ventral surface of the body, about half-way
between its two ends, is the anus or' cloaca! aperture {an),
and on either side of it a small hole, the abdominal pore,
opening into the coelome. From the end of the snout to
the last gill-cleft is considered as the head of the fish ; from
the last gill-cleft to the anus as the trunk ; and the rest as
the tail.
A longitudinal streak (/. 1 ) on each side of the body, con-
nected in front with a series of branching lines on the head
and continued backwards to the tail, is known as the lateral
XXIX
EXOSKELETON
369
line. The whole apparatus, together w ith other canals in the
head, is really a system of tubes sunk in the skin, and con-
stitutes an important, but imperfectly understood, sensory
organ.
Springing from the body are a number of flattened folds,
called the fins^ divisible into median and paired. The
median folds are two dorsal fins (d,f. ly d.f. 2) along the
middle line of the back, a caudal fin (cd, f) lying mostly
along the ventral edge of the upturned tail, and a ventral
fin (v. f) behind the anus. The paired folds are the pectoral
fins {pct,f)^ situated one on each side of the trunk just
behind the last gill-cleft, and the pelvic finsy one on each
side of the anus. The pectoral and pelvic fins are the
paired appendages or limbs of the dogfish : as in other
Vertebrates there are only two pairs, the pectorals corre-
sponding w'ith the fore-limbs, the pel vies with the hind
limbs of the higher forms.
The fish swims by vigorous strokes of the tail : the
pectoral fins are used chiefly for steering, and the dorsal
and ventral fins serve, like the keel of a boat, to maintain
equilibrium.
The skin or external layer of the body-wall consists, as
usual, of two layers, an outer layer of deric epithelium
(Fig. 99, Der. Epthm) differing from that of previous types
in being formed of several layers of cells, and , an inner
layer of connective tissue, the dermis. In the dermis are
innumerable close-set calcareous bodies (Fig. 99, Derm, Sp),
each consisting of a little irregular plate of bone produced
into a short enamelled spine, which projects through the
epidermis and gives a rough, sand-paper-like character to
the skin. These placoid scales or dermal teeth together
constitute the exoskeleton of the dogfish: it is a discon-
tinuous dermal exoskeleton like that of the starfish.
Beneath the dermis is the muscular layer in which we
Fig. 99. — Diagraiiimatic scciions of
LESS. XXIX
GENERAL STRUCTURE
371
A, longitudinal vertical section.
B. horizontal section through the pharynx and gills.
c, transverse section through the trunk.
The ectoderm is dotted, the nervous system finely dotted, the endo-
dcrm radially striated, the mesoderm evenly shaded, the ccelomic
epithelium represented by a beaded line, and all skeletal stntctures
black.
The body gives origin to the dorsal (Z>. D. F^\ ventral ( V, F\
and caudal ( C. F) fins ; the paired fins are not shown.
The body-wall consists of deric epithelium {Der. Epthm)^ dermis
(Derm\ and muscle {M) : the latter is mctamerically segmented and is
very thick, especially dorsally, where it forms half the total vertical
height (c).
The exoskeleton consists of calcified dermal spines {Derm, Sp) in the
dermis, and of dermal fin-rays {Derm. F. R) in the fins.
The endoskeleton consists of a row of vertebral centra ( V. Cent) below
the spinal cord {Sp. Cd)^ Saving rise to neural arches {N.A)^ which enclose
the cord, and in the caudal regions to hoemal arches {II. A) : a cranium
{Cr) enclosing the brain {Br) : upper and lower jaws : branchial arches
{Br. A) and rays {Br. R^ Br. R), shown only in B, supporting the
gills : shoulder {Sh. G) and pelvic {Pelv. G) girdles : and pterygiopnores
{Ptgph) supporting the fins.
The mouth {Mth) leads into the oral cavity {Or, cav)^ from which the
pharynx {Pk) and gullet {Gul) lead to the stomach {St)i this is con-
nected with a short intestine (/«/) opening into a cloaca (C/) which
communicates with the exterior by the vent {An). The oral cavity and
cloaca are the only parts of the canal lined by ectoderm.
Connected with the enteric canal are the liver {Lr) with the gall-
bladder ( G. BI) and bile-duct {B. Z>), the pancreas (A/), and the spleen
{Spl). The mouth is bounded above and below by teeth ( 7’).
The respiratory organs consist of pouches (shown in b) communicating
with the pharynx by internal {Int. br, ap) and with the exterior by
external {Ext, br. ap) branchial apertures, and lined by mucous mem-
brane raised into branchial filaments {Br. Fit).
The heart (///) is ventral and anterior, and is situated in a special
compartment of the coelome {Ped), Six of the most important blood-
vessels, the dorsal vessel (dorsal aorta, D. .Ao), the cardinal veins
{Card. F), the lateral vessels (lateral veins, Lat. V), and the ventral
vessel (intra-intestinal vein, I. int. V) are shown in c.
The whole coelome is lined by epithelium, showing parietal {Ccel.
Epthm) and visceral {C<ei. Eptkm') layers.
The ovaries {Ovy) are connected with the dorsal body-w.all : the
oviducts ( Ovd) open anteriorly into the coelome {ovd') and posteriorly
into the cloaca.
The kidneys {JC) are made up of nephridia {Nph) and open by ureters
( Ur) into the cloaca.
The nervous system is lodged in the cerebro-spinal cavity ( C. Sp. Cav)
hollowed out in the dorsal body-wall : it consists of brain {Br) and
spinal cord {Sp. Cd), and contains a continuous cavity, the neurocoele
(«. cee),
B E 2
372
THE DOGFISH
LESS. XXIX
meet, for the first time in our present subject, with distinct
metameric segmentation. The muscles are divided into
segments or myomeres (Fig. 98, myni) following one another
from before backwards, and having a zigzag disposition.
The fibres composing them are longitudinal, and are inserted
at either end into fibrous partitions or myocommas (myc),
which separate the myomeres from one another. The mus-
cular layer is of great thickness, especially its dorsal portion
(Fig. 99, c). The fibres of all the body muscles are of the
striped kind.
There is a large cailome (Fig. 99, Co*/), remarkable for being
confined to the trunk, both head and tail being, in the adult,
accelomate. The cavity is divisible into two parts : a large
abdominal cavity^ containing most of the viscera, and a small
anterior and ventral compartment, the pericardial cavity {Pcd)^
containing the heart. Both are lined by coelomic epithelium
(Cad, Epthni)^ underlain by a layer of connective tissue,
a strong lining membrane being thus produced, called peri-
tonenm in the abdominal, in the pericardial cavity.
Another very characteristic feature is that the dorsal body-
wall is tunnelled, from end to end, by a median longitudinal
neural cavity^ in which the central nervous system is con-
tained. The greater part of the cavity is narrow and cylin-
drical, and contains the spinal cord : its anterior or cerebral
portion is dilated, and contains the brain.
Imbedded in the body-wall and extending into the fins
are the various parts of the endoskelefon. This characteristic
supporting framework is formed of a tough elastic tissue
called cartilage or gristle, more or less impregnated with
lime salts, so as to have, in part, the appearance of bone.
As, however, the hard parts of the dogfishes skeleton have a
different microscopic structure from the bones of the higher
vertebrates, they are best described as mlcified cartilage.
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'P J3
rs s
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374
THE DOGFISH
The entire skeleton consists of separate pieces of cartilage,
calcified or not, and connected with one another by sheets
or bands of connective tissue called ligaments : it is divisible
into the following parts : —
A. The sknll or skeleton of the head, consisting of —
1. The cranium or brain-case, enclosing the brain
and the chief sense-organs.
2. The upper and hnver jatvs,
3. The visceral arches^ a series of cartilaginous hoops
supporting the gills.
n. The vertebral column or backbone,” a jointed axis ex-
tending from the cranium to the end of the tail,
and enclosing the spinal cord.
c. The skeleton of the median Jins.
D, The skeleton of the paired fins, consisting of —
1. The shoulder-girdle or pectoral arch^ to which are
attached
2. The pectoral Jins.
3. The hip-girdle or pelvic arch^ to which are at-
tached
4. The pelvic Jins.
The cranium (Fig. 100, O) is an irregular cartilaginous
box containing a spacious cavity for the brain, and pro-
duced into two pairs of outstanding projections : a posterior
pair, called the auditory capsules {aud. cp\ for the lodgment
of the organs of hearing, and an anterior pair, the oljactory
capsules (plj. cp\ for the organs of smell. Between the
olfactory and auditory capsules, on each side, the cranium
is hollowed out into an orbit (or) for the reception of the
eye. In front the cranium is produced into three cartila-
ginous rods (r), which support the snout. On its posterior
XXIX
BRANCHIAL ARCHES
375
face is a large aperture, the foramen ma^i^num^ through which
the brain joins the spinal cord, and on each side of the
foramen is an oval elevation or condyle for articulation with
the first vertebra.
In the human and other higher vertebrate skulls the
upper jaw is firmly united to the cranium, and the lower
alone is free. But in the dogfish both jaws {up, y, /. j) are
connected with the cranium by ligament {Ig^ Ig) only, and
each consists of strong paired (right and left) moieties,
united with one another by fibrous tissue, 'fhe posterior
end of the upper jaw presents a rounded surface, on which
fits a corresponding concavity on the lower jaw, so that a
free articulation is produced, the lower jaw working up and
down in the vertical plane, not from side to side like the
jaws of the crayfish.
The visceral arches consist of six pairs of cartilaginous
half-hoops, lying in the walls of the pharynx (Fig. 99, b,
Br, A)y and united with one another below so as to form a
basket-like apparatus supporting the gills. The first of these
arches is distinguished as the hyoid^ and is situated imme-
diately behind the jaws. It consists of two parts, a strong,
rod-like hyomandibular (Fig. 100, hy, m), which articulates
above with the auditory capsule, and is connected below by
fibrous tissue with the jaws, thus helping to suspend them
to the cranium : and a byoid cornuj which curves forwards
inside the lower jaw, and is connected with its fellow of the
opposite side by a median plate which supports the tongue.
The remaining five arches (br, a, i — br, a, ^ are called the
branchial arches. Each is formed of several separate pieces^
movably united by fibrous tissue so as to render possible the
distension of the throat during swallowing. Both they and
the hyoid give attachment to delicate cartilaginous branchial
rays {br, r, br, / : Fig. 99, Br, K) which support the gills.
376
THK doc; KISH
LESS.
The vertebral eolunin lias the general charactei of a
jointed tube surrounding the spinal portion of the neural
canal. Lying beneath this cavity, />., between it and the
coelome, is a longitudinal row of biconcave discs, the ver-
Fig. ioi. — a, Thi\e trunk vcrtcbrx of Scyllium from the side.
B, a single trunk vertebra viewed from one end.
c, three caudal vertebree from the side.
1), a single caudal vertebra from one end.
< , centrum ; h. hmmal arch ; n, a, neural arch ; tr, pr, transverse
pioces'^,
(After Hasse.)
tebral centra (Fig. loi, c; Fig. 99, V. Cent): they are
formed of cartilage, but have their anterior and posterior
faces strongly calcified. I'he biconcave intervals between
them (Fig. 99, a) are filled with a gelatinous matter or inter-
XXIX
VERTEBRAL COLUMN
377
vertebral substance. The centra arc united by ligament, so
that the whole chain of discs is very flexible. Connected
with the dorsal aspect of the series of centra is a cartila-
ginous tunnel, arching over the spinal cord : it is divided
into segments, corresponding with, but usually twice as
numerous as the centra, and called the neural arches
(Fig. loi, n, a; Fig. 99, N, A),
In the anterior part of the vertebral column the centra
give off paired outstanding processes (Fig. loi, a and n,
tr, pr) called transverse processes^ to the end of each of
which is articulated a short cartilaginous rod, the rib.
Further back the transverse processes are directed down-
wards, instead of outwards, and in the whole caudal or tail
region they unite below, forming Juemal arches (Fig. loi, c
and D, h, a ; Fig. 99, a, H, A\ which together constitute a
kind of inverted tunnel in which lie the artery and vein of
the tail. In the region of the caudal fin the haemal arches are
produced into strong median hcemal spines (Fig. 99, a, II, A
to the right), which act as supports to the fin. A centrum,
together with the corresponding neural arch and transverse
processes or haemal arch, forms a vertebra or single segment
of the vertebral column.
It should be noticed that in the vertebral column we
have another instance of the metameric segmentation of the
vertebrate body. The vertebrae do not, however, correspond
with the myomere.s, but alternate with them. The myo-
commas are attached to the middle of the vertebrae, so that
each myomere acts upon two vertebrae and thus produces
the lateral flexion of the backbone.
In the embryo, before the development of the vertebral
column, an unsegmented gelatinous rod, the notochord^ lies
beneath the neural cavity in the position occupied in the
adult by the line of centra, by the development of which it
378
THE DOGFISH
LESS.
is Largely rt'pLu'ed, Much of it, however, remains as the
gelatinous interverteljral substance, 'hhe notochord is one
of the most characteristic organs of the WTtel)rata.
'Fhe skeleton of the mediati fins consists of a series of
parallel cartilaginous rods, the fin-rays or p/erygiopkores
Fig. 102 . — Ventral view of pectoral arch of Scylliuni with right
]K'ctoral fin.
Th(‘ pet tor.il arch is divisible into dorsal g) and ventral {pit.
portitnis sepai.ded iiy the arlicnlar facets {art. f) for the fin.
The peeloial fin is foiined of three ba'^ai eartiiagt'^ {fis. I -3) and
numerous latlials (on/) ; its fieeedgc is suppttitetl liy ilennal lays (k. f. r).
(Modified from Marshall and Hurst.)
(Fig. 99, Ptgph)^ the proximal ends of which arc more or
less fused together to form basal (\artilages or inisa/ia. d'he
free edges of the fins are supported by a double series of
delicate horn-like fibres, the dermal fin-rays {Derm. F. R).
The shoulder-girdle (Fig. 102) is a strong, inverted arch of
XXIX
TEETH
379
calcified cartilage, situated just behind the last branchial
arch (Fig. 99, A, S/i, G), On each side of its outer surface
it presents three elevations or articular facets (Fig. 102, art,f)
for the pectoral fin ; the presence of these allows of the divi-
sion of each side of the arch into a narrow, pointed dorsal
portion {pct,g\ and a broader ventral portion {petg) united
in the middle line with its fellow of the opposite side. The
pectoral fin is formed of pterygiophores {rad), fused proxim-
ally to form basals {Bs, i. — 3), which are three in number,
and very large and strong. •
The pelvic girdle is a transverse bar of cartilage situated
just in front of the vent (Fig. 99, A, Felv, G)i and present-
ing on its posterior edge facets for the pelvic fin. The latter
has the same general structure as the other fins, but has a
single very large basal cartilage, and its first or anterior
radial is also much enlarged. The free edges of both
pectoral and pelvic fins are supported by horn-like dermal
rays (Fig. 102, dfr).
It will . be noticed that while the skeleton of the crayfish
is a series of articulated tubes with the muscles inside
them, that of the dogfish is a series of articulated rods with
the muscles outside. The joints, formed by two rods
applied at their ends and bound together by ligament, are
not confined to movement in one plane, like the hinge-
joints of the crayfish, but are capable of more or less
rotatory movement.
The mouth, as we have seen, is a transverse aperture
bounded by the upper and lower jaws. In the mucous
membrane covering the jaws are imbedded large numbers
of teeth, (Fig. 99, T) bony conical bodies, with enamelled
tips, arranged in transverse rows. They are to be looked
upon as special developments of the placoid scales or dermal
teeth, enlarged for the purpose of seizing prey.
Fig. 103, — Dissection of Scyllium canicuia from the left side. The left side of the body-wall is cut away to the
median plane so as to expose the abdominal {oM. c<tv)y pericardial {pcd, cav) and neural [n. cav) cavities in their
whole length.
LESS. XXIX
ENTERIC CANAI.
381
In the skeleton the cartilaginous parts are dotted, the bony ends of
the vertebra; black, f//, centra ; n. neural arches ; //. ha;mal
arches ; rr, cranium ; r, rostrum ; u. j\ upper jaw ; /. /, lower jaw ;
b. hy, basi-hyal, supporting tongue (//<^) ; b. br^ basi-branchial ; pit. a,
pectoral arch ; pv. <2, pelvic arch. The front part of the cranium is
roofed by a membranous fontanelle {fon).
The enteric canal with the liver (/. /r, r. Ir), Sic. , has been displaced
downwards, and the oral cavity and pharynx {p/i), part of the intestine
{tnt)t and the cloaca (r/) have been opened, sp^ spiracle ; t. br. a'-/, br. a\
internal branchial apertures ; cd, st^ cardiac, and py/. st, pyloric portions
of stomach ; sp. 57/, spiral valve of intestine (int) ; /. /r, left, and r. /r,
right lol)e of liver ; pa»f pancreas ; sp/, spleen ; ret. j^/, rectal gland ;
mes, mesentery.
The heart consists of sinus venosus (r. v), auricle {nu), vpntricle {y),
and conus arteriosus {e. art) : the latter gives off the ventral aorta {v, ao)
from which are seen to arise the afferent branchial arteries of the right
side. The dorsal aorta {d. ao) receives anteriorly the efferent branchial
arteries, and posteriorly becomes the caudal artery {ed. a), lying above
the caudal vein {cd. v).
The spinal cord {sp. cd) passes in front into the brain, which consists
of medulla oblongata {mod, obi), cerebellum (erb), optic lobes {opt. /), dien-
cephalon {dien), proscephalon (prs), and olfactory lobes {olf. /). To the
diencephalon are attacned the pineal {pin) and pituitary (^/y) bodies.
The left kidney (Xv/) opens by the ureter {ur) into the ufinogcnital
sinus (//. g. 5) which discharges into the cloaca. The left spermary {ts)
is connected with the epididymis (opid) from which the vas deferens
{zf. def) passes backwards, dilates into the vesicula seminalis {vs. sem)
and opens into the urinogenital sinus, with which is also connected the
sperm-sac {sp. s). Attached to the fold of peritoneum supporting the
liver is a small tube {p, n. d) representing the oviduct of the female.
The mouth (Pigs. 99, Mth and 103) leads into an oral
cavity (Or. cav), which passes insensibly into the throat or
pharynx {ph), a division of the enteric canal distinguished
by having its walls perforated by five pairs of slits, the in-
ternal branchial apertures (/. hr. a 1-5) as well as by the inner
opening of the spiracle (sp). The pharynx is continued by a
short gullet (gut) into a capacious U-shaped stomach consist-
ing of a wide cardiac division (cd. st) and a narrow pyloric
(pyt st) division. The pyloric division communicates by a
narrow valvular aperture with the intestine (int\ a wide,
nearly straight tube having its lining membrane produced
into a spiral fold, the spiral valve (sp. vl)^ which practically
THE DOGFISH
LESS.
382
converts the intestine into a very long, closely-coiled tube,
and greatly increases the absorbent surface. Finally the
intestine opens into a large chamber, the cloaca (c/)y which
communicates with the exterior by the vent.
From the gullet backwards the enteric canal is contained
in the abdominal division of the coelome, to the dorsal wall
of which it is suspended by a median mesentery (Fig. 99,
c, and Fig. 103, wes). The greater part of the canal is de-
veloped from the enteron of the embryo, and is consequently
lined by endoderm ; only the oral cavity is formed from
the stomodaeum, and the cloaca from the proctodaeum (Fig.
99, a). Outside the enteric epithelium are connective and
muscular layers, the latter formed of unstriped fibres : it is
generally characteristic of Vertebrates that the voluntary
muscles are striped, the involuntary unstriped.
I'he digestive glands are characteristic. The largest is an
immense /iver (Fig. 99, Zr) divided into two lobes (Fig.
103, /. /r, r. Ir) and situated below the stomach along the
whole length of the abdomen, to the wall of which it is
attached by a fold of peritoneum. It discharges its secretion,
the biky into the commencement of the intestine by a tube,
the bile-duct (Fig. 99, B, jy)y which gives off a blind offshoot
terminating in a large sac, the gall-bladder {G. Bl) ; this
serves as a reservoir for the bile, the chief function of which
is to act upon the fatty portions of the food. But besides
secreting this special digestive juice, the liver-cells produce
a substance called glycogen or animal starch, which is passed
directly into the blood in the form of sugar.
Another gland, of considerably smaller size, is the pan-
creas (Fig. 99, Bn Fig. 103, pan) ; it lies against the
anterior end of the intestine, into which it opens by the
pancreatic duct. It secretes pancreatic juiccy which has an
action upon all the principal classes of food, converting
XXIX GILLS 383
proteids into peptones, starch into sugar, and breaking up
fats. Opening into the cloaca is a small finger-like reda/
gland {ref, gl\ the function of which is uncertain.
In addition to these glands the inner surface of the
stomach and intestine is dotted all over with microscopic
apertures, leading into minute tubular glands sunk in the
mucous membrane. These are the gastric and intestinal
glands : the former secrete gastric juice, which digests pro-
teids ; the latter intestinal juice, which probably acts upon all
classes of food. Thus as compared with the animals pre-
viously studied, the dogfish, in common with other Verte-
brates, shows an extraordinary differentiation of digestive
glands and fluids.
There is another characteristic vertebrate organ in close
connection with the enteric canal and called the spleen
(spl). It is an irregular dark-red, gland-like body, of con-
siderable size, attached by peritoneum to the stomach. It
has no duct, and its chief function is probably the manufac-
ture of leucocytes and the disposal of worn-out red blood
corpuscles. Other ductless glands arc the thyroid in the
throat \ the thymus in connection with the dorsal ends of
the branchial arches ; and the supra- and inter-renal bodies in
relation with the kidneys.
The respiratory organs or gills consist of five pairs of
pouches, each opening by one of the internal branchial
apertures (Figs. 99, a and u. Ini, br, ap and 103) into the
pharynx, and by one of the external branchial apertures
{Ext, br, ap) on the exterior. The walls of the pouches arc
supported by the visceral arches {Br, A) and branchial rays
{Br, E, Br, B!), and are lined with mucous membrane
raised into horizontal ridges, the branchial filaments {Br, Fit),
which are abundantly supplied with blood-vessels, and are
the actual organs of respiration. As the fish swims, water
384
THE DOGFISH
LESS. XXIX
enters the mouth and passes by the internal clefts into the
branchial pouches, and thence outwards by the external clefts,
a constant siqiply of oxygen being thus ensured. The gill-
pouches are developed as offshoots of the pharynx, and the
respiratory epithelium is therefore endodermal, not ecto-
dermal, as in the starfish, crayfish, and niu.ssel.
The organs of circulation attain a degree of specialisation
not met with in any of our former types. I'lie heart is
situated in the pericardial cavity or anterior compartment of
the ccelome, and is a large muscular organ composed of four
chambers. Posteriorly is a small, thin-walled sinus venosus
(Figs. 103 and 104, s. v\ opening in front into a capacious
thin-walled auricle (an) \ this communicates with a very thick-
walled ventricle (v\ from which is given off in front a tubular
chamber, also with thick muscular walls, the anius arteriosus
(c. art). There are valves between the sinus and the
auricle, and between the auricle and ventricle, and the conus
contains three longitudinal rows of valves : all the valves
are arranged so as to allow of free passage of blood from
sinus to auricle, auricle to ventricle, and ventricle to conus,
but to prevent any flow in the opposite direction. The
heart, alone among the involuntary muscles, is formed of
striped fibres.
The conus gives off in front a single blood-vessel (^. ao\
having thick elastic walls composed of connective and elastic
tissue and unstriped muscle. This vessel, the ventral aorta^
passes forwards beneath the gills, and gives off on each side
paired lateral branches, the afferent branchial arteries
(a. br. a). Each afferent artery passes to the corresponding
gill, and there branches out into smaller and smaller arteries,
which finally become microscopic, and open into a network
of delicate tubes called capillaries.^ with which the connec-
tive tissue of the branchial filaments is permeated. The
386
THE DOGFISH
LESS.
capillaries, unlike the arteries, have no muscle or connective
tissue in their walls, which are formed of a single layer of
epithelial cells. The blood in these respiratory capillaries
is therefore brought into close relation with the surrounding
water, and as the blood flows through them it exchanges its
carbon dioxide for oxygen.
From the respiratory capillaries the blood is collected into
minute arteries, which join into larger and larger trunks, and
finally unite into efferent bramhial arteries {ebr. a — two to
each gill — by which the purified blood is carried from the
gills. The efferent arteries of the right and left sides unite
in a median longitudinal artery, the dorsal aorta (d. ao\
which passes backwards, immediately beneath the vertebral
column, to the end of the tail.
From the efferent branchial arteries and the dorsal aorta
are given off numerous arteries supplying the whole of the
body with blood. The most important of these are two pairs
of carotid arteries (r. a) to the head, a pair of subclavians
{scl. a) to the pectoral fins, unpaired codiac (cl. a) and n/es-
enteric arteries (ms. a) to the enteric canal, liver, pancreas,
and .spleen, numerous paired renals (r. a) to the kidneys,
spermatics (sp. a) to the gonads, and a pair of iliacs (il. a)
to the pelvic fins. The posterior part of the dorsal aorta,
supplying the tail, is contained in the hsemal canal of the
caudal vertebree, and is known as the caudal artery (cd. a).
All these arteries branch and branch again in the various
parts to which they are distributed, their ultimate ramifica-
tions opening, as in the case of the gills, into a capillary
network with which every tissue, except the cartilages and
the epithelia, is permeated.* In traversing these systemic
capillaries the blood parts with its oxygen and various
nutrient matters to the tissues, and receives from them
carbon dioxide and other waste matters.
XXIX
CIRCULATORY GROANS
>^7
From the systemic as from the respiratory capillaries the
blood is collected into vessels which join into larger and
larger efferent trunks. But these trunks are not thick-walled
elastic arteries, but thin-walled, non-elastic, collapsible tubes,
having valves at intervals, called vetfis. As a general rule
every part of the body has a vein running alongside its
artery, the blood in the artery flowing to the part in
([uestion, that of the vein away from it.
The blood from the head is brought back by a pair of
^ugitlar veins ( /. v) : each of these enters a large precaval
vein {pr, nj. v\ which passes vertically downwards and
enters the sinus venosus. The blood from the tail is re-
turned by a caudal vein (cd, 7i) lying immediately beneath
the caudal artery in the haemal canal : this vessel enters the
coelome and then divides into right and left branc:hes, the
renal portal veins {r, /. v\ which pass to the kidneys and
join with the capillaries of these organs, the impure blood
brought from the tail mingling with the pure blood of the
renal arteries {r. a). From the kidneys the blood is returned
into a pair of immense cardinal veins (erd, v), which pass
forwards, receiving veins from the reproductive organs (sp, v),
muscles, &c., and finally join each with the corresponding
jugular to form the precaval vein.
From the stomach, intestine, spleen, and pancreas the
blood is collected by numerous veins, which all join to form
a large hepatic portal vein (h, /. v). This behaves in the
same way as the renal portal : instead of joining a larger
vein on its way to the heart, it passes to the liver and breaks
up to connect with the capillaries of that, organ, its blood,
deprived of oxygen but loaded with nutrient matters from
the enteric canal, mingling with the oxygenated blood
brought to the liver by a branch of the cadiac artery. After
circulating through the capillaries of the liver the blood
c c 2
388 THE DOGFISH less.
is taken by a pair of hepatic veins (k. v) to the sinus
venosus.
The iliac veins {il. v) from the pelvic fins pour their blood
into the lateral veins {lat, v), paired trunks running forwards
in the side walls of the body to the sinus venosus, and
receiving at their anterior ends the subclavian veins (scl. v)
from the pectoral fins.
Some of the veins, especially the cardinals and spermatics,
are dilated into spacious cavities called sinuses. These are,
however, of a totally different nature from the sinuses of the
crayfish, which are mere spaces among the tissues devoid of
proper walls. In the dogfish, as in Vertebrata generally, the
blood is confined, throughout its course, to definite vessels,
the heart, arteries, capillaries, and veins invariably forming
a closed system of communicating tubes.
The general course of the circulation will be seen to agree
with that already described in the crayfish and mussel : />.,
the blood is driven by the contractions of the heart through
the arteries to the various tissues of the body, whence it is
returned to the heart by the veins or sinuses (compare
Figs. 88, 96, and 104 a). But whereas in both crayfish
and mussel the respiratory organs are interposed in the
returning current, both their afferent and efferent vessels
being veins, in the dogfish they are interposed in the out-
going current, their afferent and efferent vessels being
arteries. An artery, it must be remembered, is a vessel
taking blood from the heart to the tissues of the body, and
having thick walls td resist the strain of the heart’s pulsa-
tion ; a vein is a thin-walled vessel bringing back the blood
from the tissues to the heart.
Moreover, the circulation in the dogfish is complicated by
the presence of the two portal systems, renal and hepatic.
In both of these we have a vein, renal portal or hepatic
XXIX
CIRCULATION
389
portal, which, instead of joining with larger and larger veins,
and so returning its blood directly to the heart, breaks up,
after the manner of an artery, in the kidney or liver, the
blood finding its way into the ordinary venous channels
after having traversed the capillaries of the gland in question.
Thus an ordinary artery arises from the heart or from an
Fig. 104A.— Diagram illustrating the course of the circulation in the
Dogfish.
Vessels containing oxygenated blood, red ; non*oxygenated, blue.
j 5, capillaries of the body generally ; of the enteric canal ; 6’. of
the gills ; A', of the kidneys ; Z, of the liver ; 7] of the tail.
a, br. fl, afferent branchial arteries , auricle ; r. a, conus arteri-
osus ; d, aOi dorsal aorta ; e, br. efferent branchial arteries ; h. p. Vy
hepatic portal vein ; h. Vy hepatic vein ; /r, lacteals ; fyy lymphatics ;
pr. cv. Vy precaval vein ; r. p. v, renal portal vein ; s. Vy sinus venosus ;
Vy ventricle ; v. aoy ventral aorta.
The anows show the direction of the current.
(From Parker and IlaswelPs Zoology.)
artery of higher order and ends in capillaries ; an ordinary
vein arises from a capillary network and ends in a vein of
higher order or in the heart. But the hepatic and renal
portal veins end in capillaries after the manner of arteries,
and the efferent branchial arteries begin in capillaries after
the manner of veins.
THE DOOFISII
LESS.
390
VV^itli regard to the general morphology of the blood-system,
the dorsal aorta with the caudal artery may be considered as
a dorsal vessel (compare l^olygordius, p. 279, and Crayfish,
[). 340), the caudal vein, hepatic portal vein, heart, and ven-
tral aorta as together representing a ventral vessel, the affer-
ent and efferent branchial arteries as commissural vessels,
and the lateral veins as lateral vessels. It will be seen that
the heart of Vertebrates is a muscular dilatation of the
ventral vessel.
The blood is red, the colour being due, as in some species
of Polygordius (p. 280), to hiemoglobin. The pigment is not,
however, diffused in the plasma of the blood, but is confined
to the red corpuscles^ flattened oval colls with large nuclei,
like those of the frog referred to in an early Lesson (p. 56,
f'ig. 8), Among the red corpuscles, but in much smaller
numbers, are leucocytes. When the blood is fully oxy-
genated it takes on a bright scarlet colour, and is usually
called arterial blood ; when the oxygen has been given up to
the tissues the colour becomes dull purple, and the blood is
called venous. But the student must avoid the common
error that arterial blood is necessarily confined to arteries
and venous to veins ; in the dogfish, for instance, the ventral
aorta and the afferent branchial arteries contain venous
blood.
In addition to the blood-vessels the dogfish possesses a
set of channels called lymphatics (Fig. 104A, ly\ consisting of
colourless thin-walled vessels, mostly running alongside the
arteries and veins. Traced in one direction they ramify ex-
tensively, and end in a set of lymph-capillaries interwoven
with, but distinct from, the blood- capillaries ; traced in the
other direction they join into larger and larger trunks, pro-
vided at intervals with valves, and finally open into the
veins. The lymph- capillaries take up the drainage from the
XXIX
NERVOUS SYSTEM
391
tissues and pass it into the veins. The fluid they contain,
called lymphy is practically blood, minus its red corpuscles;
its leucocytes are formed in structures called lymphatic giandSy
which occur in the course of the vessels. The lymphatics
of the enteric canal are called lacieals ; they take an im-
portant share in the absorj)tion of fats.
The nenums systeniy like the circulatory organs, is vastly
in advance of anything we have yet met with. 'Fhe central
nervous system consists of a brain (Fig. 103), contained
in the cranial cavity, and (xnitinuous posteriorly with a
spinal cord (sp, c) contained in the neural canal of the back-
bone. Thus the central nervous system is exclusively dorsal
in position, and is not traversed by the enteric canal as in
Polygordius, the crayfish, and the mussel.
Another characteristic feature of the dogfish’s nervous
system is that it is not solid, like that of Polygordius and
the crayfish, but is tubular, being traversed by a longitudinal
canal, the neurocade (Fig. 99, N. Ca:\ lined with epithelium.
In the spinal cord the neuroccele has the form of a narrow
central canal ; in the brain it exjiands into a fairly capacious
system of cavities, the cerebral ventricles.
The brain or anterior expan.sion of the nervous system is
a complex structure divisible into several parts. The hind-
most division, continuous with the spinal cord, is the medulla
oblongata (Figs. 103 and 105, NH)y and has above it the cere-
bellum {HH). Immediately in front of these two divisions is
the mid-brainy produced above into paired elevations, the optic
lobes. In front of the mid-brain is a small section called
the diencephalon {ZH)y and anterior to this again a large
prosencephalon ( VH)y corresponding with the cerebral
hemispheres of the higher Vertebrata, and giving off in front
paired olfactory lobes (L. ol). All these divisions of the
brain contain ventricles ( 7 ^. rho)y varying considerably in
392
THE DOGFISH
LESS.
form and size. Connected with the dorsal region of the
,W ■ Mm$
Fk'.. 105. —D(ii sal view i,i. .nA\no{i\\GScyllit4fncanicula.
"1 lie posterior division of tlie l)rain is the medulla oblongata (iV//),
on the (loisa! suiface of which is shown one of the cerebral ventricles
(/^
The huge cerebellum (////) nearly covers the optic lobes (J///).
The (liencephalon (///) sliows in the middle one of the cerebral ven-
tricles, and the place of attachment of the pineal stalk [Gp).
riu' juosenceplialon { V/I) gives off the olfactor}' lobes {'J'ro, L. oL).
'The follov.'ing neives are shown : — ojitic (//), trochlear (/F’), tri-
geminal ( /bb facial ( F//), auditory ( glossopharyngeal {IX)^ and
vagus
(From Wicdersheim.)
diencephalon is the pineal body (Fig. 103, /m), representing
XXIX
NERVES
393
the vestige of a sensory organ, and connected with the ventral
surface of the same division is the pituitary body {pty\
The mode of origin of the nerves is also characteristic.
From the spinal cord the nerves arise segmen tally, one pair
corresponding to each myomere, and pass through aper-
tures in the neural arches of the vertebrae. Each arises by
two roots, a dorsal and a ventral. The dorsal root is dilated
into a ganglion, and contains only sensory fibres ; the ventral
root is non-ganglionated, and is motor. A longitudinal
ganglionated sympathetic nervCy extending along the dorsal
region of the coelome, is connected with the spinal nerves,
and sends branches to the viscera, blood-vessels, &c.
From the brain arise ten pairs of nerves, some of which
are sensory, others motor, others mixed. Three are the
nerves of the principal sense-organs, the first or olfactory
supplying the organ of smell ; the second or optic (Fig. 105,
ii) the retina of the eye {see below), and the eighth or auditory
( VIII), the organ of hearing. The third or oculomotor, the
fourth or trochlear {IV), and the sixth or abducent go to the
muscles of the eye ; the fifth or trigeminal {V) to the snout
and jaws ; the seventh or facial { VII) to the palate, lower
jaw, and hyoid arch ; the ninth or glossopharyngeal {IX) to
the hyoid and first branchial arches, and the tenth or vagus
to the remaining branchial arches, as well as to the heart,
stomach, and lateral line.
Besides the sensory tubes of the skin (lateral line, &c.),
which are probably the seat of a delicate tactile sense, and the
rudimentary tongue, which is presumably an organ of taste,
there are three pairs of sensory organs, the structure and posi-
tion of which is very characteristic of Vertebrates. These
are the olfactory organs, the eyes, and the auditory organs.
The olfactory organs are a pair of cup-like sacs on the
under side of the snout, enclosed in the olfactory capsules
394
THE DOGFISH
LESS.
and opening externally by the nostrils. They are lined with
mucous membrane, which is raised up into ridges so as to
increase the surface. The actual organ of smell is the
epithelium forming the superficial layer of the mucous
membrane ; it is developed as an in-pushing of the ectoderm,
and is supplied by the olfactory nerve.
The eyes are a pair of nearly globular organs, lying in the
orbits and moved each by six muscles. Their structure is,
in all essential respects, the same as in man. There is an
outer capsule, the sckrotk^ formed of cartilage, lined by a
vascular membrane, the choroid^ within which is a delicate
membrane, pigmented externally, the retina or actual organ
of sight. In the front or exposed part of the eye the
sclerotic passes into a transparent, watch-glass-like cornea^
and the choroid into a curtain or diaphragm, the />/>, having
a central aperture, the piipil^ to admit the rays of light to
the interior of the eye. Behind the pupil is a gelatinous,
biconvex crystalline lens of glassy transparency. I'he
space between the cornea and the iris is called the aqueous
chamber of the eye, and is filled by a watery fluid, the
aqueous humour. The main part of the cavity of the eye,
bounded in front by the lens, and for the rest of its extent
by the retina, is the vitreous chamber^ and is filled with a
gelatinous substance, the vitreous humour. The cornea,
aqueous humour, lens, and vitreous humour together form a
series of adjustable lenses serving to focus objects on the
retina, and the stimulus thus applied to that membrane is
conveyed by the fibres of the optic nerve to the brain.
The auditory organ is a sac of complex form, the mem-
branous labyrinth^ enclosed in the auditory capsule of the
skull, where it floats in a watery fluid, the perilymph. It
consists of a sac called the vestibule^ with which are con-
nected three tubes, called from their form the semicircular
XXIX
URINOGENITAL ORGANS
39S
canals. Two of these, the anterior and posterior canals, are
vertical in position, and are united with one another at their
adjacent ends ; at the other end each is dilated to form a
bulb-like swelling, the ampulla. 'Ehc third or horizontal
canal opens at each end into the vestibule, and has an
ampulla at its anterior end. The vestibule gives off a tube,
the endolymphatic duct^ which opens on the top of the head.
I'he whole ai)paratus contains a fluiti, the endolymplty in
which is a gelatinous substance enclosing calcareous par-
ticles or otoliths. Patches of sensory ci)ithelium are found
in the vestibule and in the ampulke, and to these the fibres
of the auditory nerve are distributed. There seems little
doubt that the membranous labyrinth has not only an
auditory, but also an ecjuilibrating function — />., that the fish
is enabled by its means to maintain its equilibrium in the
water, as is also the case with the statocysts of Invertebrates.
The excretory and the reproductive organs of the dogfish
are so closely associated as to be spoken of together as the
urino^enital organs. The sexes arc distinct, and the males
are distinguished externally by having a pair of large grooved
rods, the claspersy connected with the inner borders of the
pelvic fins. They are used, like the peculiarly modified finst
and second pairs of pleopods in the male crayfish (p. 323),
as copulatory organs.
The kidneys (Fig. 103, kd) are long, flat, lobulated bodies
lying one on each side of the backbone in the posterior
part of the abdominal cavity. From the ventral surface of
each spring numerous delicate ducts or ureters (ur\ .some
of which unite, opening into a small unpaired chamber,
called in the female the urinary sinus and in the male the
urinogenital sinus {u. g. s)y which opens into the cloaca.
In the embryo the kidneys appear in the form of separate
segmentally arranged tubes (Fig. 99, JVph) having the
396
THE DOGFISH
general character of nephridia, opening on the one hand
by nephrostomes into the coelome, and on the other into a
longitudinal duct which discharges into the cloaca. Thus
the primitive structure of the kidney furnishes another
instance of metamerism in the dogfish.
In the male there is a single pair of spermarks (Fig. 103,
ts\ in the form of large soft organs, united with one another
posteriorly. They are suspended by a fold of peritoneum
to the dorsal body-wall. From the anterior end of each
arise numerous delicate efferent ducts, which enter a long,
convoluted spermiduct or vas deferens (v, def). This passes
along the ventral aspect of the kidneys and dilates into a
conical pouch, the vesicula seminalis {vs, sem), and the two
vesiculae open, along with the ureters and a pair of reser-
voirs called sperm-sacs {sp, s), into the urinogenital sinus.
The female has a single ovary (Fig. 99, ovy) suspended to
the dorsal body-wall by a fold of peritoneum. In the adult it
is studded all over with rounded projections, the ova, varying
in diameter from 12-14 nim. downwards. The oviducts {ovd)
are paired and extend along the whole length of the dorsal
wall of the coelome, below the kidneys. Anteriorly they
unite with one another below the gullet and just in front of
the liver, and at the point of junction is a single aperture of
considerable size {ovd), by which both tubes communicate with
the coelome : posteriorly they open into the cloaca. About
the anterior third of each oviduct is narrow ; its posterior
two-thirds is wide and distensible, and at the junction of the
two parts is a yellowish, glandular mass, the skell-gland.
Internal impregnation takes place, the spermatic fluid of
the male being passed, by means of the claspers, into the
oviducts of the female. The eggs, when ripe, break loose
from the surface of the ovary into the coelome, and thence
jxiss, through the common aperture, into one or other of
XXIX
DEVELOPMENT
397
the oviducts, where fertilisation occurs. As it passes into
the dilated portion of the oviduct the oosperm of Scyllium
becomes surrounded by a horn-like egg-shell or “ mermaid’s
purse ” secreted by the shell-gland, and having the form of
a pillow-case produced at each of its four angles into a long,
tendril-like process. The eggs are laid among .sea- weed, to
which they become attached by their tendrils. In Acanthias
and Mustelus a mere vestige of the egg-shell is formed, and
the eggs undergo the whole of their development in the
Fig. io6. — Section of the upper part of the embryo of a Dogfish in
the blastula stage.
The blastoderm is formed of a single layer of ectoderm cells (white)
and of several rows of cells (shaded), which subsequently give rise to
endoderm and mesoderm ; sg., the blastocoele.
Below the blastoderm is the unsegmented yolk containing scattered
nuclei in).
(From Balfour.)
oviducts, the young being eventually born alive with the
form and proportions of the adult.
The great size of the egg is due to the immense quantity
of yolk it contains : its protoplasm is almost entirely aggre-
gated at one pole in the form of a small disc. When
segmentation of the oosperir takes place it affects the
protoplasm alone, the inactive yolk, as in the Crayfish
(p. 344), taking no part in the process. The polyplast
stage consequently consists of a little heap of cells, called
the blastoderm (Fig. 106), at one pole of an undivided
39 «
THE DOGFISH
1,KSS.
sphere of yolk. I'he edge of the blastoderm beeunies
invaginatcd at one point (gaslrula-stagc*, p. 295) and its cells
become difirn ntiatt^d into the three embryonic layers —
ectoderm, m- od. nn, and endoderni. At the same time
the blastoderm extends in a peri[)heral direction so as
gradually to <'o\er the yolk, and its middle part becomes
raised up into a rulge like tbi('kening which is moulded,
F ic.. 107. — A, embryo of Scyllium with yolk-sac (x ih): B, underside
of head, eiilat^ed
br. /, branchial filaments ]>rotrudin^ tiiroujab gill-clcfts ; br. f\
branchial filaments jiioliiidinj^ throui^h spiracle ; id. caudal fin ; d, /,
ilorsal fins ; c, eyt‘ ; c.v. br. external branchial apertures ;
moiUli ; iiostiiis; pit. f\ pectoral fin ; in\ /, pelvic fin; st, yolk-
slalk ; r' /, ventral fin ; yk. yolk-sac,
(After Balfour, slightly altered.)
step by step, into the form of the embryo fish. The head,
trunk, and tail actpiire distinctness, and become more and
more clearly separated off from the bulk of the egg, tlie
latter taking the form of a yolk-sac (Fig. 107, a, yk. s)
attached by a narrow stalk to the ventral surface of the
embryo.
In this condition the various parts of the adult fish ran
XXIX
DEVELOPMENT
399
be recognized, but the proportions are different, and the
head presents several peculiarities. The gill-filaments
(br, f) are so long as to project through the external
branchial apertures and the spiracle {br, f\ in the form of
long threads, abundantly supplied with blood-vessels, and
apparently serving for the absorption of nutriment — the
albumen in the egg-shell in the case of Scyllium, secretions
of the oviduct in the viviparous forms. Besides this mode
of nutrition the yolk-sac communicates with the intestine by
a narrow duct {st\ through which absorption of its contents is
constantly going on. liy the time the young fish is ready
to be hatched or born the greater part of the yolk-sac has
been drawn into the cielome, a mere vestige of it still
dangling from the ventral surface of the body.
LESSON XXX
MOSSES
In the six previous lessons we have traced the advance
in organization of animals from the simple diploblastic
Hydra to the complicated triploblastic forms which con-
stitute the five higher phyla of the animal kingdom. We
have now to follow in the same way the advance in structure
of plants. The last member of the vegetable kingdom with
which we were concerned was Nitella (Lesson XX), a solid
aggregate, exhibiting a certain differentiation of form and
structure, but yet composed of what were clearly recogniz-
able as cells, there being, as in Hydra, none of those
well-marked tissues which form so noticeable a feature in
Polygordius as in other animals above the Coelenterata.
Taking Nitella as a starting point, we shall see that among
plants, as among animals, there is an increasing differentia-
tion in structure and in function as we ascend the series. The
first steps in the process are well illustrated by a considera-
tion of that very abundant and beautiful group of plants, the
Mosses. In spite of the variations in detail met with in
different genera of the group, the essential features of their
organization are so constant that the following description
will be found to apply to any of the common forms.
402
MOSSES
LESS.
c, semi’diagrammatic vertical section of a moss, showing the arrange-
ment of the tissues. The stem is formed externally of scleivnchyma
/), and contains an axial bundle {ax. b ) : in some of the leaves {/)
tiie section passes through the mitlrib, in others (/') through the lateral
portion : the stem ends distally in an apical cell (a/>. r), from which
segmental cells r) are separated.
D, transverse section of the stem of Rryum roseum, showing scleren-
chyma (^r/), axial bundle {ax. and rhizoids {rh). ( x 6o.)
E, transveise section of a leaf of Funaria^ showing the midrib {md. r)
formed of several layers of cells, anrl the lateral portions one cell thich.
F, small portion of the lateral region of the same, showing the form
of the cells and the chromatophores {chr). ( x 150.)
distal end of the stem of Fonttmilis antif>yreitca in vertical section,
.showing the apical cell {ap. r) giving rise to segmental cells (.?<;'•. r),
which by subserpient <Uvision form the segments of the stem with the
leaves ; the thick lines show the boundaries of the segments.
n, diagram of the apical cell of a moss in the form of a tetrahedron
with rounded base abc and three flat sides abd^ bed^ acd.
(d, after Sachs ; G, after Leitgeb.)
The plant consists of a short slender stem (Fig. 108, a, st\
from which are given off structures of two kinds, rhizoids or
root-hairs {rh\ which pass downwards into the soil, and leaves
(/), which are closely set on the stem and its branches. As
in Nitella (p. 205) the portion of the stem from which a leaf
arises is called a node, and the |)art intervening between any
two nodes an internodc, while the name segment is applied
to a node with the internode next below it. At the upper or
distal end of the stem the leaves are crowded, forming a
terminal bud.
Owing to the opacity of the stem, its structure can only be
made out by the examination of thin section.s (c and d). It is
a solid aggregate of close-set cells which are not all alike, but
exhibit a certain amount of differentiation. In the outer
two or three rows the cells {set) are elongated in the direc-
tion of the length of the stem, so as to have a spindle-shape,
and their walls are greatly thickened and of a reddish colour.
They thus form a protective and supporting tissue, to which
the name sekremhyma is applied. Running longitudinally
XXX TERMINAL BUD 40;^
through the centre of the stem is ii mass of tissue (tiA'. />)
distinguished by its small, thin-walled cells, and constituting
the axia/ ln 4 ndle.
The leaves (u) are shaped like a spear-head, pointed
distally, and attached proximally by a broad base to the
stem. The axial portion (b and k, md. r, c, /) consists ot
several layers of somewhat elongated cells and is called the
midrib : the lateral portions (k and v : c*, /) are formed of a
single layer of short cells. Thus the leaf has, for the most
part, tlie character of a superficial aggregate, 'riie cells
contain oval chromatophores (f, c/ir),
'The rhizoids (c and d, r/i) are linear aggregates, being
formed of elongated cells, devoid of chlorophyll, arranged
end to end.
In the terminal bud the leaves, as in Nitella (pp. 206 and
208), arch over the growing point of the stem, which in this
case also is formed of a single apical cell (c and o, a/>, c).
But in correspondence with the increased complexity of the
plant, the apical cell is not a hemisphere from which new
segments are cut off parallel to its flat base, but has the form
(h) of an inverted, three-sided pyramid or tetrahedron, tlu*
rounded base of which (abc) forms the apex of the stem
while segments {seg. c) are cut off from each of its three
triangular sides in succession.
The best way to understand the apical growth of a moss
is to cut a tetrahedron with rounded base out of a carrot or
turnip : this represents the apical cell (h) : then cut off a
slice parallel to the side abd, a second parallel to bed, and a
third parallel to acd: these represent three successively
formed segments. Now imagine tliat after every division
the tetrahedron grows to its original size, and a very fair
notion will be obtained of the way in which the successive
segments of the moss-stem are formed by the fission in three
D D 2
404
MOSSES
LESS.
planes of the apic'al cell. Each segment (c and o, sej^. c)
immediately after its separation divides and subdivides, pro-
ducing a mass of cells from which a i)rojection grows out
forming a leaf, and in this way the stem increases in length
and the leaves in number.
Asexual reproduction takes place in various ways ; all of
them are, however, varieties of budding, and the buds always
arise in the form of a linear aggregate of cells called a
protonema : from this the moss-plant develops in the same
way as from the j)rotonema arising from a spore (p. 408).
The gonads are developed at the extremity of the main
stem or one of its branches, and are enclosed in an involucre
or tuft of leaves often of a reddish colour — the terminal bud
of the fertile shoot or so-called “ flower ” of the moss.
The spermary (Mg. 109, a^, a^) is an elongated club-
shaped body consisting of a solid mass of cells, the outer-
most of which form the wall of the organ, while the inner
(a^) become converted into sperms. The latter (a'*) are
spirally coiled and provided with two cilia : they are liber-
ated by the rupture of the wall of the spermary at its
distal end (a^), and swim in the rain or dew covering the
plant.
The ovaries ' (see Preface, p. viii) (b^, b®) may or may
not occur on tlie same plant as the spermaries, some
mosses being monoecious, others dioecious. Like the sperm-
aries, they consist at first of a solid mass of cells which
assumes the form of a flask, having a rounded basal portion
or venter (z;) and a long neck (//). The outer layer of cells
in the neck and the two outer layers in the venter form the
wall of the ovary, the internal cells are arranged in a single
' The ovary of mosses, ferns, &c., is usually called an archegonium :
the spermary, as in the lower plants, an antheridium.
XXX
DEVELOPMENT OF SPOROGONIUM
405
axial row at first similar to those of the wall. As the ova’ry
develops, the proximal or low^ermost cell of the axial row
takes on the character of an ovum (b^, ozf) ; the others, called
cafia/ cells {cn, c), are converted into mucilage, which by its
expansion forces open the mouth of the flask and thus makes
a clear passage from the exterior to the ovum (b®).
Through the passage thus formed a sperm makes its way
and conjugates with the ovum, producing as usual an
oosperm or unicellular embryo.
The development of the embryo is at first remarkably
like what we have found to take place in Hydroids (p. 246).
The oosperm, having surrounded itself with a cell-wall,
divides into two cells by a wall at right angles to the long
axis of the ovary : each of these cells divides again re-
peatedly, and there is produced a solid multicellular embryo
or polyplast (c^, spgnm).
Very early, however, the moss-polyplast exhibits a striking
difference from the animal polyplast or morula : one of its
cells — that nearest the neck of the ovary — takes on the
character of an apical cell, and begins to form fresh seg-
ments like the apical cell of the stem. 'J'hus the plant
embryo differs almost from the first from the animal
embryo. In the animal there is no apical cell : all the
cells of the polyplast divide and take their share in the
formation of the permanent tissues. In the plant one cell
is at a very early period differentiated into an apical cell, and
from it all cells thereafter produced are, directly or indirectly,
derived.
The embryo continues to grow, forming a long rod-like
body (c^, spgmn) the ba.se of which becomes sunk in the
tissue of the mo.ss-stem, while its distal end projects vertically
upwards, covered by the distended venter {v) of the ovary.
Gradually it elongates more and more and its distal end
LESS. XXX
PROTONEMA
407
A*, a sperm-cell with enclosed sperm : A*, a free-swimming sperm.
(x8oo.)
B^, an ovary of Funaria, surface view, showing venter (7^) and neck
(;i) : B®, the same in optical section, showing ovum (<w) and canal cells
{cn. c) : B®, the same after disappearance of the canal cells : the neck is
freely open, and the ovum {ov) exposed. ( x 200. )
ovary with withered neck containing an embryo in the
polyplast stage ( x 200) : in c- the ovary, consisting of swollen venter (?^)
and shrivelled neck («), encloses a young sjwrogonium ; the
distal end of the stem is shown with bases of leaves (/) ; in c^the venter
has ruptured, forming a proximal portion or sheath and a distal portion
or calyptra which is carried up by the growth of the st)orogonium.
O'*, a small plant of Funaria with ripe sporogonium consisting of seta
(s/), with urn («) and lid (/) covered by the calyptra (r).
c®, diagrammatic vertical section of urn («), snowing lid (/), air spaces
(«), and spores (s/).
d\ a germinating spore of Funaria, showing ruptured outer coal (s^)
and young protonema (/^) with rhizoid (r//). ( x 550.)
D-, portion of protonema of the same, showing lateral bud (M), from
which the leafy plant arises. ( x 90.)
(a and D, after Sachs j B, c', and c®, altered from Sachs. )
dilates : the embryo has now become a sporogonium, con-
sisting of a slender stalk (c^ st) bearing a vase-like capsule
or urn (u) at its distal end. In the meantime the elonga-
tion of the stalk has caused the rupture of the enveloping
venter of the ovary (c^) ; its proximal part remains as a sort
of sheath round the ba.se of the stalk, while its distal portion,
with the shrivelled remains of the neck («), is carried up by
the elongation of the .sporogonium and forms an extinguisher-
like cap or calyptra (c^ c) over the urn.
As development goes on, the distal end of the urn be-
comes separated in the form of a lid (c*, /), and certain
of the cells in its interior, called spore-mother cellSy divide
each into four daughter cells, which acquire a double cell-
wall and constitute the spores (c®, sp) of the moss.
When the spores are ripe the calyptra falls off or is blown
away by the wind, the lid separates from the urn, and the
spores are scattered.
In germination, the protoplasm of the Sj[)ore covered by
4o8
MOSSES
l.ESS.
the inner layer of the cell-wall protrudes through a split in
the outer layer (d\ s/>) and grows into a long filament, the
protonema {pr,\ divided by oblique septa into a row of cells,
'rhe protonema — which it will be observed is a simple linear
aggregate — branches, and may form a closely-matted mass
of filaments. Sooner or later small lateral buds (d-, bd)
af)pear at various places on the protonema : each of these
takes on the form of a three-sided pyramidal apical cell,
which then proceeds to divide in the characteristic way
(p. 403), forming three rows of segments from which leaves
spring. In this way each lateral bud of the protonema gives
rise to a moss-plant.
Obviously we have here a somewhat complicated case ot
alternation of generations (see p. 248). I'he gamobium or
sexual generation is represented by the moss-plant, which
originates by budding and produces the sexual organs, while
the agamobium consists of the sporogonium, developed from
the oosperm and reproducing by means of spores, 'fhe
protonema, arising from a spore and producing the leafy
plant by budding, is merely a stage of the gamobium.
I'he nutrition of mosses is holoph^tic ; but there is a
striking differentiation of function correlated with terrestrial
habits. In Isitella the entire organism is submerged in
water and all the cells contain chlorophyll, so that decom-
position of carbon dioxide and absorption of an aqueous
solution of salts are performed by all parts alike, every
cell being nourished independently of the rest. In the
moss, on the other hand, the rootlets are removed from
the influence of light and contain no chlorophyll : hence
they cannot decompose carbon dioxide ; but, being sur-
rounded by moist soil, are in the most favourable position
for absorbing water and mineral salts. The stem, again, is
XXX
DISTRIBUTION OF FOOD-MATERIALS
409
converted into an organ of support : the thickness of its
external cells prevents absorption and it contains no
chlorophyll. Hence the function of decomposing carbon
dioxide is confined to the leaves.
We have thus as an important fact in the nutrition of an
ordinary terrestrial plant that its carbon is taken in at one
place, its water, nitrogen, sulphur, potassium, &:c., at another.
But as all parts of the plant require all these substances it is
evident that there must be some means by whic'h the root
can obtain a supply of carbon, and the leaves a sup])ly of
elements other than carbon. In other words, we find for
the first time in the ascending series of plants, just as we
did in ascending from the simple Hydra to the complex
Polygordius (p. 278) the need for some contrivance for the
distribution of food-materials.
The way in which this distributing process is performed
has been studied chiefly in the higher plants, but its essential
features are probably the same for mosses.
Water is continually evaporating from the surface of the
leaves, its place being as constantly supplied by water — with
salts in solution — taken in by the rhizoids. 'Phis trtnis-
piratioHy or giving off of water from the leaves, is one
important factor in the process under consideration, since
it ensures a constant upwar<l current of water, or, more
accurately, of an aqueous solution of mineral salts. The
withering of a plucked moss-plant is of course due to the
fact that when the roots are not embedded in moist soil or
in water, transpiration is no longer balanced by absorption.^
In the higher plants it has been found that the root-hairs
have an absorbent action independent of transpiration, so
that water may be absorbed in the absence of leaves.
^ Mosses, however, unlike most higher plants, can absorb water by
their leaves.
410
MOSSES
LESS.
By the transpiration current, then, the leaves are kept
constantly supplied with a solution of mineral salts derived
from the soil, and are thus nourished like any of the aquatic
green plants considered in previous lessons : by the double
decomposition of water and carbon dioxide a carbo-hydrate
is formed : this, by further combination with the nitrogen
of the ai)sorbed ammonium salts or nitrates, forms simple
nitrogenous compounds, and from these, probably through
a long series of mesostates or intermediate products, proto-
l)lasm is finally manufactured.
In this way the food supply of the green cells of the
leaves is accounted for, but we have still to consider that of
the colourless cells of the stem and rhizoids, which, as we
have seen, arc supplied by the transpiration current with
everything they require except carbon, and this, owing to
their possessing no chlorophyll, they are unable to take in
in the form of carbon dioxide.
As a matter of fact the chlorophyll-containing cells of the
leaves have to provide not only their own food, but also
that of their not-green fellows. In addition to making good
the waste of their own protoplasm they produce large
(Quantities of plastic products (see p. 33) such as grape
sugar, and simple nitrogenous compounds like asparagin,
and these pass by diffusion from cell to cell until they reach
the uttermost parts of the plant, such as the axis of the
stem and the extremities of the rhizoids. The colourless
cells are in this w'ay provided not only with the salts
contained in the ascending transpiration current, but with
carbo-hydrates and nitrogenous compounds. From these
they derive their nutriment, living therefore like yeast-cells
in Pasteur^s solution, or like Bacteria in an organic
infusion.
We see then that the colourless cells of the stem and
XXX
DISTRIBUTION OF FOOD-MATERIALS
411
rhizoids are dependent upon the green cells of the leaves
for their supplies. Like other cells devoid of chlorophyll
they are unable to make use of carbon dioxide as a source
of carbon, but require ready-made carbo-hydrates, the
manufacture of which is continually going on, during
daylight, in the chlorophyll-containing cells of the leaves.
This striking division of labour is the most imi)ortant
physiological difference between mosses and the more lowly
organised green plants described in previous lessons.
LESSON XXXI
FERNb
We saw in tlie previous lesson that in mosses there is a
ctTtain though small amount of histological differentiation,
some cells being modilied to form sclerenchyma, others to
form axial bundles. We have now to consider a group of
plants which may be considered to be, in this respect, on
much the same morphological level as Folygordius, the
adult organism being composed not of a mere aggregate of
simple cells, but of various well-marked tissues.
A fern- plant has a strong stem which in some forms, such
as the common Bracken {Pteris aquilina) is a horizontal
underground structure called a rhizome^ often incorrectly
considered as a root : in others it creeps over the trunks of
trees or over rocks : in others again, such as the tree-ferns,
it is vertical, and may attain a height of three or four metres.
From the stem are given off structures of two kinds — the
leaves, which present an almost infinite variety of form in
the various species, and the numerous slender roots. In
some cases, such as the tree-ferns and the common Male
Shield-fern {Aspidium fiHx-mas\ the plant ends distally in a
terminal bud, consisting, as in Nitella and mosses, of the
growing end of the stem over-arched by leaves : in others
tESS. XXXI
TISSUES OF THE STEM
4^3
such as Pteris, the stem ends in a blunt, knnb-likc extremity
(}uite uncovered l)y leaves. On the proximal portion of the
stem are usually found the withered remains of the leaves
of previous seasons, or the scars left by their fill, 'i'he
roots are given off from the whole surface of the stem,
often covering it with a closely-matted mass of dark brown
fibres.
When the stem is cut across transversely (P'ig. no, a) it
is seen, even with the naked eye, to consist of three well
marked tissues. The main mass of it is formed of a whitish
substance, soft and rather sticky to the touch, and ('ailed
}(round~pareHchyma {par ) : this is covered by an external
layer of very hard tissue, dark brown or black in colour, the
hypodermis {hyp ) : bands of a similar hard brown substance
are variously distributed through the parenchyma, and con-
stitute the sclerenchyma {scl ) : and interspersed with these
are rounded or oval patches of a yellowish colour {V.B)
harder than the parenchyma but not so hard as the
sclerenchyma, and called vascular bundles.
The general distribution of these tissues can be made out
by making longitudinal sections of the stem in various
planes or by cutting away the hypodermis, and then scraping
the parenchyma from the vascular bundles and bands of
sclerenchyma. The hypodermis is found to form a more or
less complete hard sheath or shell to the stem, while the
sclerenchyma and vascular bundles form longitudinal bands
and rods imbedded in the parenchyma, and serve as a sort
of supporting framew'ork Or skeleton.
The minute structure of the stem can be made out by
the examination either of very thin longitudinal and trans-
verse sections, or of a bit of stem which has been reduced
to a pulp by boiling in nitric acid with the addition of a few
crystals of potassium chlorate ; by this process the various
LESS. XXXI
GENERAX. CHARACTERS
415
A, Transverse section of the stem of Pterin aqiiilina^ showing hyix^-
derinis {hjp)^ grouncl-parencliynia(/Vr/'), sclei eiichynui (j< /), and vascular
bundles!/: Af). (x2.)
B, tmns verse section of a vascular bundle, showing bundle-sheath
(A j//), sieve-tubes {sv, /), scalariform vessels (sr. 7 >), and spiral vessels
{sp, v). (x6.)
c, senii-diagrammatic vertical section of the growing point of the
stem, showing apical cell (ap, < ), segmental cells {sep. r), anti apical
meristem (ap. mcr) passing into permanent tissue consisting of epidermis
{ep)y hypodermis {}iyp\ ground parenchyma (/tr?*), sclcrenchyma
and vascular bundles in which the sheath ih. sh)^ sieve-tubes (m /),
scalariform vessels {sr. v)^ and spiral vessels {sp. 7 >) are intlicated.
p, a single parenchyma cell, showing nucleus (/;//), and vacuoh*
(?w).
E, cell of hypodermis.
F, portion of a sieve-tube, showing sievc-jdales {s?\ //).
G, portion of a spiral vessel with the spiral fibre partly unrolled at the
lower end.
H, fibre-like cell of sclcrenchyma.
I, portion of a scalariform vessel, part of the wall being supposed to
be removed.
K, vertical section of a leaf of Pteris, showing upper and lower epi-
dermis {ep)y mesophyll cells {ms. ph)y with intercellular spaces {i, c. sp),
a stoma (j/) in the lower epidermis, and hairs {k).
L, surface view of epidermis olT leaf of Aspidium, showing two stomata
(r/) with their guard-cells {pd. c),
M, vertical section of the end of a root, showing apical cell {ap. r),
segmental cells {ssp. c), and root-cap {r. cp) with its youngest cap-cells
marked cp, c.
(A, B, and D-K after Howes 5 M from Sachs, slightly altered. )
tissue elements are separated from one another, and can be
readily examined under a high power.
By combining these two methods of sectioning and
dissociation, the parenchyma is found to consist of an
aggregate of polyhedral cells {q, par ; p) considerably longer
than broad, their long axes being parallel with that of the
stem itself. The cells are to be considered as right cylinders
which have been converted into polyhedra by mutual pres-
sure. They have the usual structure, and their protoplasm is
frequently loaded with large starch-grains. They do not fit
quite closely together, but spaces are left between them,
especially at the angles, called intercellular spaces.
4i0
FERNS
LESS.
The cells of the hxpOclermjbiL (e) are proportionally longer
than those of the parenchyma, and are pointed at each end :
tliey contain no starch. Their walls are greatly thickened*
and are composed not of cellulose but of lignin, a carbo-
hydrate allied in composition to cellulose, but containing a
larger proportion of carbon. Schulze^s solution, which, as
we have seen, stains cellulose blue, imparts a yellow colour
to lignin.
Outside the hypodermis is a single layer of cells (c, e/>)
not distinguishable by the naked eye and forming the actual
external layer of the stem : the cells have slightly thickened,
yellowish-brown walls, and constitute the ^ide nnis . From
many of them are given off delicate filamentous processes
consisting each of a single row of cells : these are called
hairs.
In the sclerenchyma the cells (h) are greatly elongated,
and pointed at both ends, so as to have the character rather
of fibres than of cells. Their walls are immensely thickened
and lignified, and present at intervals oblique markings due to
narrow but deep clefts : these are produced by the deposition
of lignin from the surface of the protoplasm (see p. 33) being
interrupted here and there, instead of going on continuously
as in the case of a cell-wall of uniform thickness.
The yascular bundles have in transverse section (b) the
appearance of a very complicated network, with meshes of
varying diameter. In longitudinal sections (c) and in dis-
sociated specimens they are found to be partly composed of
cells, but to contain besides structures which cannot be
called cells at all.
In the centre of the bundle are a few narrow cylindrical
tubes (b and c, sp, v,) characterised at once by a spiral
marking, and hence called spiral vessels. Accurate exam-
ination shows that their walls (o) are for the most part thin.
XXXI
XYLKM AND PHLOKM
4»7
but are thickened by a spiral fibre, just as a pn])er tube
might be strengthened by gumming a spiral strip of i)aste-
board to its inner surface. These vessels are of considerable
length, and are open at both ends : moreover they contain
no protoplasm, but are filled with either air or water : they
have therefore none of the characteristics of cells. They
are shown, by treatment with Schulze’s solution, to lie com-
posed of lignin.
Surrounding the group of spiral vessels, and forming the
large polygonal meshes so obvious in a transverse section,
are wide tubes (b and c, sc. v) pointed at both ends and
fitting against one another in longitudinal series by their
oblique extremities. They havO transverse markings like
the rungs of a ladder, and are hence called scnhiriform
vessels. The markings (i) are due to wide transverse pits
in the otherwise thick lignified walls : in the obliciue ends
by which the vessels fit against one another the pits are
frequently replaced by actual slits, so that a longitudinal
series of such vessels forms a continuous tube containing,
like the spiral vessels, air or water, but no protoplasm. In
most ferns the terminal walls are not thus perforated, and
the elements aite then called tracheides.
The presence of these ve.ssels —spiral and scalariform —
is the most important histological character separating ferns
and mosses. The latter group and all plants below them are
composed exclusively of cells: ferns and all plants above
them contain vessels in addition, and are hence called vas-
cular plants.
The vessels, together with small parenchyma-cells inter-
spersed among them, make up the central portion of the
vascular bundle, called the wood or xyle?n. The peripheral
portion is formed of several layers of cells composing the dast
or phloem^ and surrounding the whole are two layers of
E £
4i8
FERNS
LESS.
small cells, the inner called the phloems heath or perievile, the
outer, the Inmdle-sheath or endodermis {Ik sh).
I’he cells of the i)hloem are for the most part parenchy-
matous, but among them are some to which special
attention must be drawn. These (b and c, sv, /), are many
times as long as they are broad, and have on their walls
irregular patches ox sieve-plates {Y^sv,pi.) composed of groups
of minute holes through which the protoplasm of the cell is
continuous with that of an adjacent cell. The transverse or
obli(jue partitions between the cells of a longitudinal series
are also perforated, so that a row of such cells forms a sieve-
tube in w'hich the protoplasm is continuous from end to end.
We have here, therefore, as striking an instance of a non-
cellular tissue as in the deric epithelium and certain other
tissues of l^olygordius (see p. 289).
The distal or growing end of the stem terminates in a blunt
apical cone or punctum vegetationis (c), surrounded by the
leaves of the terminal bud in the case of vertical stems, or
sunk in a depression and protected by close-set hairs in the
underground stem of the bracken.’ A rough longitudinal
section showjj: that, at a short distance from the apical cone,
the various tissues of the stem — epidermis, parenchyma,
sclerenchyma, and vascular bundles — merge insensibly into
a whitish substance, resembling parenchyma to the naked
eye, and called apical meristem {ap, mer).
Thin sections show that the summit of the apical cone is
occupied by a w'edge-shaped apical cell (a/, c) which in
vertical stems is three-sided like that of mosses (Fig. 108, h,
p. 401), while in the horizontal stem of Pteris it is two-sided.
As in mosses, segmental cells (seg, c) are cut off from the three
(or two) sides of the apical cell in succession, and by further
division form the apical meristem (ap, mer\ which consists
XXXI
APICAIv GROWTH
419
of small, close-set cells without intercellular spaces. As the
base of the apical cone is reached, the meristem is found to
pass insensibly into the permanent tissues, the cells near the
surface gradually merging into epidermis and hypodermis,
those towards the central region into sclerenchyma and the
various constituents of the vascular bundles, and those of
the intermediate regions into parenchyma.
The examination of the growing end of the stem shows us
how the process of apical growth is carried on in a compli-
cated plant like the fern. The apical cell is continually
undergoing fission, forming a succession of segmental cells ;
these divide and form the apical meristem, which is thus
being constantly added to at the growing end by the forma-
tion and subsequent fission of new segmental cells : in this
way the apex of the stem is continually growing upwards or
forwards, liut at the same time the meristem cells farthest
from the apex begin to differentiate ; some elongate but
slightly, increasing greatly in size, and become parenchyma
cells : others by elongation in the direction of length of the
stem and by thickening and lignification of the cell wall
become sclerenchyma cells : others again elongate greatly,
become arranged end to end in longitudinal rows, and, by
the loss of their protoplasm and of the transverse partitions
between the cells of each row, are converted into vessels —
spiral or scalariform according to the character of their walls.
Thus while the epidermis, parenchyma, and sclerenchyma
are formed of cells, the spiral and scalariform vessels are cell-
jtisionSy or more accurately cell-wall-fusions, being formed by
the union in a longitudinal series of a greater or less number
of cell-walls. It will be remembered that the muscle-plates
of Polygordius are proved by the study of development to be
cell-fusions (p. 302).
We thus see that ever)' cell in the stem of the fern was
E K 2
420
FERNS
LESS.
once a cell in the ai)ical meristeni, that every vessel has
arisen by the concrescence of a number of such cells, and
that the nieristein ct.‘lls themselves are all derived, by the
ordinary process of binary fission, from the apical cell. In
this way the concurrent processes of, cell-division, cell-
differentiation, and cell-fusion result in the production of
the various and complex tissues of the fully-formed stem.
'fhe leaves vary greatly in form in the numerous genera
and species of ferns : they may consist of an unbranched
s/tiM’ bearing a single expanded green blade: or the stalk
may be more or less branched, its ramifications bearing the
numerous subdiv isions of the blade, or pinnules.
I'he anatomy of the leaf, like that of the stem, can be
readily made out by a rough dissection. The leaf-stalk and
its branches have the same general structure as the stem,
consisting of parenchyma coated externally with epidermis
and strengthened internally by vascular bundles, which are
continuous with those of the stem. But the blade, or, in the
case of a compound leaf, the pinna, has a different and quite
peculiar structure. It is invested by a layer of epidermis
which can be readily stripped off as an extremely thin, colour-
less membrane, exposing a soft, green substance, the leaf-
parenchyma or mesophyll. The leaf is marked externally by
a network of delicate ridges, the veins ; these are shown by
dissection to be due to the presence of fine white threads
which ramify through the mesophyll, and can be proved by
tracing them into the leaf-stalk to spring from its vascular
bundles, of which they are in effect the greatly branched
distal ends.
Microscopic examination shows the epidermis of the leaf
(Fig. no, K, ^/and l) to consist of flattened, colourless cells
of very irregular outline and fitting closely to one another like
XXXI
LEAVES AND ROOTS
421
the parts of a child’s puzzle. Among them are found at
intervals pairs of sausage-shaped cells c) placed with
their concavities towards one another so as to bound a
narrow slit-like aperture (j/). These apertures, which are
the only intercellular spaces in the eiiiderniis, are called
stomates : the cells bounding them are the ^uardcells^ and
are distinguished from the remaining epidermic cells by the
possession of a few chromatophores.
The mesophyll, which as we have seen oc('upies the whole
space between the upjier and lower epidermis, is formed of
thin-walled cells loaded with chromatophores (k, ms.pJi) and
therefore of a deep green colour. 'The cells in contact with
the upper epidermis are cylindrical, and are arranged verti-
cally in a single row : those towards the lower surfacti are
very irregular both in form and arrangement. Large inter-
cellular spaces (/. c. sp) occ'ur between the mesophyll -ct*lls
and communicate with the outer air through the stomates.
The leaves arise as outgrowths of the distal or growing
end of the stem, each originating from a single segmental
cell of the apical cone.
The fern is the first plant we have yet considered which
possesses true roots, the structures so-called differing funda-
mentally from the simple rhizoids of Nitella and the mosses.
Instead of being mere linear aggregates of cells, they agree
in general structure with the stem from which they spring,
consisting of an outer layer of epidermis within which is
parenchyma strengthened by bands of sclerenchyma and by
a single vascular bundle in the middle. The epidermic cells
give rise to unicellular prominences, the root-hairs.
The apex of the root, like that of the stem, is formed oi
a mass of meristem in which a single wedge-shajjcd apical
cell (Fig. no, m, ap, c) can be distinguished But instead
422
FERNS
LESS.
of the base of this cell forming the actual distal extremity,
as in the stem (compare c), it is covered by several layers of
cells which constitute the root-cap (r, cp). In fact the apical
cell of the root divides not only by planes parallel to its
three sides, but also by a plane parallel to its base, and in
this way produces not only three series of segmental cells
c) wliich afterwards subdivide to form the apical
meristem, but also a series of cap-cells (cp, c) which form a
protective sheath over the tender growing end of the root as
it forces its way through the soil.
Roots are also peculiar in their development. Instead of
being, like leaves, prominences of the superficial tissues of
the stem, they arise from a layer of cells immediately ex-
ternal to the vascular bundles, and in growing force their
way through the superficial portion of the stem, through a
fissure from which they finally emerge. They are thus said
to be endogenous in origin while leaves are exogenous.
The nutrition of ferns is carried on in much the same
way as in mosses (see p. 408). Judging from the analogy of
flowering plants it w’ould seem that the ascending current ot
water from the roots passes mainly through the xylem of the
vascular bundles, while the descending current of nitrogenous
and other nutrient matters for the supply of the colourless
cells of the stem and roots passes chiefly through the phloem
and especially through the sieve-tubes. The absorption of
water is effected by the root-hairs.
In the autumn there are found on the under surfaces of
the leaves brown patches called soriy differing greatly in
form and arrangement in the various genera, and formed of
innumerable, minute, seed-like, bodies, the sporangia (Fig.
Ill, a), just visible to the naked eye. Each sorus or group
XXXI
REPRODUCTION
4»3
of sporangia is covered by a fold of the epidermis of the
leaf, called the indusium,
A sporangium is attached to the leaf by a multicellular
stalk (st\ and consists of a sac resembling two watch-glasses
placed with their concave surfaces towards one another and
their edges united by a thick rim (an). The sides are
formed of thin flattened cells with irregular outlines, the
rim or annulus of peculiarly shaped cells which are thin and
broad at one edge (to the left in a), but on the other (to the
right) are thick, strongly lignified, and of a yellowish-brown
colour. The whole internal cavity is filled with spores
(u, sp) having the form of tetrahedra with rounded edges,
and each consisting of protoplasm containing a nucleus, and
surrounded by a douVile wall of cellulose. A spore is there-
fore, as in mosses, a single cell.
Each sporangium arises from a single epidermic cell of
the leaf. This divides repeatedly so as to form a solid mass
of cells, of which the outermost become the wall of the
sporangium while the inner are the spore-mother-cells. The
latter divide each into four spores, as in mosses (p. 407).
As the spores ripen, the w^all of the sporangium dries, and
as it does so the thickened part of the annulus straightens
out, tearing the thin cells and producing a great rent through
which the spores escape (b).
When the spores are sown on moist earth they germinate,
the protoplasm, covered by the inner coat, protruding
through the ruptured outer coat (c, sp) in the form of a
short filament This divides tran.sversely, forming two cells,
the proximal of which sends off a short rhizoid (rh\ The
resemblance of this stage to the young protonema of a moss
is sufficiently obvious (see Fig. 109, p. 406).
Further cell-division takes place, and before long the
Fig. III. — Reproduction and Development of Ferns.
A, Sporangium of Fieris^ external view, showing stalk (r/) and
annulus {mi).
B, the same, during dehiscence, the spores {sp) escaping.
c, a germinating sjwre, showing the ruptured outer coat {$p\ and a
THE PROTHALLUS
LESS. XXXI
425
rhizoid [rh) springing from the proximal cell of the rudimentary (two-
cel led) prothallus.
D, a young prothallus, showing spore, rhizoid, apical cell («/. f),
and segmental cells {seg. c).
E, an advanced prothallus, from beneath, showing rhizoids (r//),
ovaries (ov}')t and spermaries (spj').
F, a mature spermary of J*Uris, inverted with its distal end
directed upwards) so as to compare with Fig. 109, A.
0, a single sperm, showing coiled body and numerous cilia.
II, a mature ovary of Aspiditim^ inverted so as to compare with Fig.
109, iJ®, showing venter (z>), neck (w), ovum (e?*), and canal cells {m. r).
1, small portion of a prothallus of Aspknium in vertical section,
showing the venter {v) and part of the neck («) of a single ovary after
fertilisation. The venter contains an embryo just passing from the
polyplast into the phyllula stage, and divided into four groups of cells,
the rudiments respectively of the font {ft)^ stern (j/), rout (;/), and
cotyledon (r/).
K, vertical section of a prothallus {prth) of Ncphrolepis^ bearing
rhizoids {rh)y and a single ovary with greatly dilated venter (») and
withered neck (;/). The venter contains an embiyo in the phyllula
stage, consisting of foot (//), rudiments of stem (s/), and root (; /), and
cotyledon (r/) beginning to grow upwards. "
L, prothallus (/r///) with rhizoids (r//), bearing a young fern-plant,
consisting of foot (//), rudiment of stem (j/), first root (r/), cotyledon
(< 7 ), and first ordinary leaf (/). (After IIowcs.)
distal cells divide longitudinally, a leaf-like body being
produced, which is called the prothallus (d). This is at first
only one layer of cells thick, but it gradually increases in
size, becoming more or less kidney-shaped (e), and as it
does so its cells divide parallel to the surface, making it two
and finally several cells in thickness. Thus the prothallus is
at first a linear, then a superficial, and ultimately a solid
aggregate. Root-hairs (rh) are produced in great number
from its lower surface, and penetrating into the soil serve
for the absorption of nutriment. At an early period a two-
sided apical cell (d, ap, c) is differentiated, and gives off
segmental cells (seg, c) in the usual way: an abundant
formation of chromatophores also takes place at a very early
period in the cells of the prothallus, which therefore re-
sembles both in structure and in habit some very simple
form of moss.
425
FERNS
LESS.
On the lower surface of the prothallus gonads (e, spy^ ovy)
are developed, resembling in their essential features those of
mosses. The spermaries {spy) make their appearance first,
being frequently found on very young prothalli. One of the
lower cells forms a projection which becomes divided off by
a septum : further division takes place, resulting in the
differentiation (f) of an outer layer of cells forming the wall
of the spermary, and of an internal mass of sperm-mother-
cells in each of which a sperm is produced. The sperm (o)
is a corkscrew-like body, probably formed from the nucleus
of the cell, bearing at its narrow end a number of cilia which
appear to originate from the protoplasm. To the thick end
is often attached a globular body, also arising from the proto-
plasm of the mother-cell ; this is finally detached.
The ovaries (e and h, oty) are not usually formed until
the prothallus has attained a considerable size. Each arises,
like a spermary, from a single cell cut off by a septum from
one of the lower cells of the prothallus : the cell divides and
forms a structure resembling in general characters the ovary
of a moss (see Fig. 109, ii, p. 406), except that the venter (h,
v) is sunk in the prothallus, and is therefore a less distinct
structure than in the lower type. As in mosses, also, an
axial row of cells is early distinguished from those forming
the wall of the ovary : the proximal of these becomes the
ovum {(w), the others are the canal-cells {cn, c)y which are
converted into mucilage, and by their expansion force open
the neck and make a clear passage for the sperm.
The sperms swarm round the aperture of the ovary and
make their way down the canal, one of them finally conju-
gating with the ovum and converting it into an oosperm.
The early stages in the development of the embryo
remind us, in their general features, of what w^e found to
occur in mosses (p. 405). The oosperm first divides by a
XXXI
POLYPLAST AND PHYLLULA
427
plane parallel to the neck of the ovary, forming two cells, an
anterior nearest the growing or distal end of the prothallus,
and a posterior towards its proximal end. Each of these
divides again by a plane at right angles to the first, there
being now an upper and a lower anterior, and an upper and
a lower posterior cell : the lower in each case being that
towards the downwardly directed neck of the ovary. Each
of the four cells undergoes fission, the embryo then consist-
ing of eight cells, two upper anterior (right and left), two
lower anterior, two upper posterior, and two lower })osterior.
We thus get a multicellular but undifferentiated stage, the
polyplast.
It will be remembered that in mosses the polyplast forms
an apical cell, and develops directly into the sporogonium
(p. 405). In the fern the later stages are more complex.
One of the upper anterior cells remains undeveloped, the
other (Fig. in, i and k, st) takes on the form of a wedge-
shaped apical cell, and, dividing in the usual way, forms a
structure like the apex of the fern-stem, of which it is in fact
the rudiment. The two upper posterior cells divide and
subdivide, and form a multicellular mass called the foot {ft\
which becomes embedded in the prothallus, and serves the
growing embryo for the absorption of nutriment. One of
the lower posterior cells remains undeveloped, the other (rt)
takes on the form of the apical cell of a root, f.^., of a wedge-
shaped cell, which not only produces three sets of segmental
cells from its sides but also cap-cells from its base (p. 422) :
division of this cell goes on very rapidly, and a primary root
is produced which at once grows downwards into the soil.
Finally the two lower anterior cells undergo rapid fission,
and develop into the first leaf of the embryo, called the
cotyledon (ct), which soon begins to grow upwards towards
the light.
428
FERNS
LESS.
l"hus at a comparatively early stage of its development
the fern-embryo has attained a degree of differentiation far
beyond anything which occurs in the moss-embryo. The
scarcely differentiated polyplast has passed into a stage
which may be called the phyllula^ distinguished by the
possession of those two characteristic organs of the higher
plants, the leaf and root.
Notice how early in development the essential features of
animal or plant manifest themselves. In Polygordius the
polyplast is succeeded by a gastrula distinguished by the
possession of a digestive cavity : in the fern no such cavity
is formed, but the polyplast is succeeded by a stage dis-
tinguished by the possession of a leaf and root. In the
one case the characteristic organ for holozoic, in the other
the characteristic organs for holophytic nutrition make their
appearance, and so mark the embryo at once as animal
or plant. We may say then that while the oosperm and
the polyplast stages of the embryo are common to the
higher plants and the higher animals, the correspond-
ence goes no further, the next step being the formation
in the animal of an enteron, in the plant of a leaf and
root. In other words the phyllula is the correlative of the
gastrula.
The cotyledon increases rapidly in size, and emerges
between the lobes of the kidney-shaped prothallus (l) : the
root at the same time grows to a considerable length, the
result being that the phyllula becomes a very obvious
structure in close connection with the prothallus, and indeed
appearing to be part of it. The two are actually, however,
quite distinct, their union depending merely upon the fact
that the foot of the phyllula is embedded in the tissue of
the prothallus like a root in the soil. Hence the phyllula
is related to the prothallus in precisely the same way as the
XXXI GAMOBIUM AND AGAMOBIUM 429
sporogoniiim to* the moss plant (compare iMg. in, K, with
Fig. 109, c^, and Fig. iii, l, with Fig. 109, c*^).
The rudiment of the stem (l, s /) continues to grow by the
production of fresh segments from its apical cell : leaves (/) are
developed from the segments, and grow upwards parallel with
the cotyledon. The leaves first formed are small and
simple in structure, but those arising later become succes-
sively larger and more complicated, until. they finally attain
the size and complexity of the ordinary leaves of the fern.
In the meantime new roots are formed and the primary root
ceases to be distinguishable ; the cotyledon, the foot, and
the prothallus wither, and thus the phyllula, by the successive
formation of ne\V parts from its constantly growing stem,
becomes a fern-plant.
We see that the life-history of the fern resembles in
essentials that of the moss. In both, alternation of genera-
tion occurs, a gamobium or sexual generation giving rise, by
the conjugation of ovum and sperm, to an agamobium or
asexual generation, which, by an asexual process of spore-
formation, produces the gamobium. But in the relative
proportions of the two generations the difference is very great.
What we know as the moss plant is the gamobium, and the
agamobium is a mere spore-producing structure, never getting
beyond the stage of a highly differentiated polyplast, and
dependent throughout its existence upon the gamobium, to
which it is permanently attached. What we know^ as the
fern plant is the agamobium, a large and complex structure
dependent only for a brief period of its early life upon the
small and insignificant gamobium. Thus while the gamobium
is the dominant phase in the life-history of mosses, the
agamobium appearing like a mere organ, in ferns the
positions are more than reversed — the agamobium may
assume the proportions of a tree, while the gamobium is so
430
FERNS
LESS. XXXI
small that its very existence is unknown to a large propor-
tion of fern-collectors.
It follows from what has just been said that the various
organs of a fern do not severally correspond with those of a
moss. I'he leaves of a moss are not homologous with those
of a fern, but are rather comparable to lobes of the pro-
thallus : in the same way the rhizoids of a moss correspond,
not with the complicated roots of the fern, but with the
rhizoids of the prothallus.
LESSON XXXII
THE CHIEF DIVISIONS OF THE VEGETABLE KINGDOM :
KgUISKTUM : SALViNIA : SELAGINKLLA
In the XXVI th Lesson (p. 304) it was pointed out that a
thorough comprehension of the structure and development
of Polygofdius would enable the student to understand the
main features of the organisation of all the higher animals.
In the same way the study of the fern paves the way to
that of the higher groups of plants, all of which, indeed, differ
far less from the fern than do the various animal forms con-
sidered in Lessons XXVI — XXIX from Polygordius. We
saw that the differences between these included matters oi
such importance as the presence or absence of segmentation
and of lateral appendages, the characters of the skeleton,
and the structure and position of the nervous system. In
the higher plants, on the other hand, the essential organs —
root, stem, and leaves — ^are, save in details of form, size, &c.,
practically the same in all: the tissues always consist of
epidermis, ground-parenchyma, and vascular bundles, the
latter being divisible into phloem and xylem ; the growing
point both of stem and of root is formed of meristem, from
which the permanent tissues arise ; and the growing point of
432
CHARACTERS OF THE HIGHER PLANTS
LESS.
the root is always protected by a root-cap, that of the stem
being simply over-arched by leaves. Moreover, an alterna-
tion of generations can be traced in all cases.
Plants may be conveniently divided into the following
chief groups or phyla :
1 . A/ga,
2. Fungi.
3. Muscinece.
4. Vascular Cryptogams.
a. Filicinae.
b. Equisetaceai.
c. Lycopodineae.
5. Phanerogams.
a. Gymnosperms.
b. Angiosperms.
The Algce are the lower green plants. They may be
unicellular, or may take the form of linear, superficial, or
solid aggregates : they never exhibit more than a limited
amount of cell-difierentiation. This group has been repre-
sented in the foregoing pages by Zooxanthella, Diatoms,
Vaucheria, Caulerpa, Monostroma, Ulva, and Nitella.
The Fungi are the lower plants devoid of chlorophyll:
some are unicellular, others are linear aggregates : in none
is there any cell-differentiation worth mentioning. Saccharo-
myces, Mucor, Penicillium, and the mushroom belong to
this group.
The position of some of the lower forms which have come
under our notice is still doubtful. Bacteria, for instance,
are considered by some authors to be Fungi, by others Algse,
while others place them in a group apart. Diatoms also art
xxxn
CHARACTERS OF THE PHYLA
433
sometimes placed in a distinct group. It must, moreover,
be remembered that most botanists include Haematococcus,
Pandorina, and Volvox among Algae, and place the Myce-
tozoa either among Fungi or in a separate group of chloro-
phyll-less plants (p. i8i).
The Muscinea are the mosses and liverworts, the former
of which were fully described in Lesson XXX.
The Vascular Cryptogams are flowerless plants in which
vascular bundles are present. Together with the Phanero-
gams they constitute what are known as vascular plants, in
contradistinction to the non-vascular Algae, Fungi, and
Muscineae, in which no formation of vessels takes place. The
group contains three subdivisions.
The first division of Vascular Cryptogams, the Filicince^
includes the ferns, an account of which has been given in
the previous lesson. It will be necessary, however, to devote
some attention to an aquatic form, called Sa/vtma, which
differs in certain important particulars from the more familiar
members of the group.
The Eguisetacece include the common horsetails (genus
Eguisetum\ a brief account of which will be given, as they
form an interesting link in their reproductive processes
between the ordinary ferns and Salvinia.
The Lycopodinece^ or club-mosses, are the highest of the
Cryptogams or flowerless plants. A short description of one
of them, the genus Selaginella^ will illustrate the most
striking peculiarities of the group.
The Phanerogams^ or flowering plants, are so called from
the fact that their reproductive organs take the form of
specially modified shoots, called cones or flowers. They are
sometimes called by the more appropriate name of Sperma-
phytes^ or seed-plants, from the fact that they alone among
plants reproduce by means of seeds structures which differ
F F
434 EQUISETUM less,
from spores in the fact that each contains an embryo plant
in the phyllula stage.
The Gymnosperms^ or naked-seeded Phanerogams, include
the cone-bearing trees, such as pines, larches, cypresses, &c.,
as well as cycads and some other less familiar forms. A
general account of this group will be given.
The Angiosperms^ or covered-seeded Phanerogams, include
all the ordinary flowering plants, as well as such trees as
oaks, elms, poplars, chestnuts, &c. A brief description of
the general features of this group will conclude the Lessons.
Equisetum
The horsetails are common British plants found usually
in moist or marshy situations, and reaching a height of
I to 3 feet.
The plant consists of a branched underground stem or
rhizome, lateral branches of which grow vertically upwards,
and constitute the aerial shoots. Both stem and branches
have a very characteristic appearance : they are distinctly
segmented or divided into nodes and internodes, and from
each node springs a crown-like structure or leapsheath
(Fig. 1 1 2, A, and Fig. 1 1 3, a, /. sk)^ formed by a whorl of
leaves united into a continuous structure. In some cases
the aerial shoots also give rise to secondary shoots (Fig. 112,
A, sK)^ arranged in whorls and apparently arising below the
leaves : actually, however, they originate in axillary buds, as
in Nitella, but, instead of growing out betw^een the stem and
the leaf, perforate the base of the latter.
The internodes of both rhizome and aerial shoots are
hollow, each having a large axial air-cavity (Fig. 112, b,
extending throughout its whole length, and formed by the
disintegration of the central parenchyma-cells of the young
XXXII
STRUCTURE
435
stem. At each node is a transverse partition separating the
internodal spaces from one another. Around the central
cavity, and corresponding with the longitudinal ribs with
which the stem is marked, is a series of smaller air-cavities
(r^)f arranged in a circle, and alternating with these, between
Fig. 1 1 2, — A, portion of aerial shoot of Eqiiisetum, showing a node
(oii) from which arise a leaf-sheath sA) and a whorl of secondary
shoots {sh). (Nat, size.)
B, transverse section of aerial shoot, showing central (r^ and peri-
pheral air-cavities, and ring of vascular bundles with smaller air-
cavities (r^). ( x 2. )
C, a single sporophyll {sp. pA) with stalk (,y/) and sporangia {sp^'-).
(xio.)
D, a single spore showing coiled elater {el).
E, the same, with elater (<?/) expanded.
(A-C, after Goebel ; D and K, after Le Maout and Decaisne. )
them and the central cavity, are the vascular bundles (z^. b\
each with a small air-cavity (r^) in its inner or central
portion.
The microscopic structure of the plant agrees in essential
respects with that of the fern, though differing in many
details to which no further reference need be made here.
F F 2
436 EQUISETUM less .
Each axis — rhizome and shoots — terminates in a tetrahedral
apical cell.
As in ferns, there is no primary root in the adult, but
numerous roots spring from the nodes of the rhizome, and
agree in all essential points of structure and development
with those of ferns.
Some of the aerial shoots bear only leaf-sheaths and
branches, and are hence called sterile shoots: others, the
fertile shoots^ terminate in a cone-like structure (Fig. 113, a ),
formed of hexagonal scales (sp. ph\ at first closely applied
to. one another at their edges, but afterwards becoming
separated. Each scale (Fig. 112 , c, and Fig. 113 , b , sp,pk)
is a mushroom-like body, springing from the axis of the cone
by a stalk {st) attached to the centre of the inner surface of
its expanded portion. Around the point of attachment of
the stalk spring from five to ten elongated sacs, the sporangia
i^pg)-
The structure and development of these mushroom-like
bodies or scales of the cone show them to be peculiarly
modified leaves, developed in whorls like the ordinary leaves
of the stem, but not cohering into sheaths, and assuming
the characteristic form just described in relation with their
special function of bearing the sporangia. We have there-
fore to distinguish, in Equisetum, between ordinary or
foliage-leaves and spore-bearing leaves or sporophylls.
The spores are developed in the same w^ay as in mosses
and ferns, but have a very distinctive structure. Outside
the usual double cell-wall is a third coat, which, as develop-
ment proceeds, becomes split up into four bands (Fig. 112,
D, E, et)y wound spirally round the spore and attached to it
by one end, the opposite expanded end being free. These
bands or elaters are hygroscopic: when moist they are coiled
round the spore (d), when dry they straighten themselves
XXXII
DIMORPHISM OF THE GAMOBIUM
437
and stand out separately from its surface (e). The spores
become entangled by their elaters, by the coiling and un-
coiling of which they are able to execute slight movements.
Fig. 1 1 3. — Reproduction and Development of Equisetiim.
A, distal end of a fertile shoot, showing two leaf-sheaths (/. sh), and
the cone formed of hexagonal sporophylls {sp. p/i). (Nat. size.)
B, diagrammatic vertical section of a portion of the cone, showing the
sporophylls {sp.ph) attached by short stalks to the axis of the cone, and
bearing sporangia {spg) on their inner surfaces.
c, a mde prothallus bearing three spermaries (j/y). ( x 100.)
D, portion of a female prothallus bearing three ovaries (^/), those to
the right and left containing ova, that in the middle a polyplast ; rht
rhizoids. ( x 30. )
(a, after Le Maout and Decaisne ; c and D, after Hofmeister.)
.The spores are liberated by the bursting of the sporangia,
and germinate, giving rise to prothalli. But instead of the
prothalli being all alike in form and size and all monoecious,
438
SALVINIA
LESS.
some (c) remain small and simple, and produce only
spermaries {sfy ) ; others (d) attain a complicated form and
a length of over a centimetre, and produce only ovaries
{ovy). I'hus although there is no difference in the spores,
the prothalli produced from them are of two distinct kinds,
the smaller being usually exclusively male, the larger female.
The oosperm develops in much the same way as in ferns :
it divides and forms a polyplast, which, by formation of a
stem, root, foot, and two cotyledons, becomes a phyllula
and grows into the adult plant.
As in the fern, the Equisetum plant, reproducing as it
does by asexual spores, is the agamobium, the gamobium
being represented by the prothalliis. The peculiarity in the
present case is that the gamobium is sexually dimorphic,
some prothalli producing only male, others only female
gonads.
Salvinia
Salvinia is a small fresh-water plant, found floating, like
duckweed, on the surface of still water.
The stem (Fig. 114, st) is an elongated slender rhizome
floating at or near the surface, and distinctly divided into
nodes and internodes. Each node gives off three appen-
dages, two broad, fiat foliage-leaves (/ /. 1-3;/ /. 1-3'),
which lie above the surface of the water, and a branched
structure {s. /. 1-3) which has all the appearance of a root,
its thread-like branches hanging down into the water and
being covered with hairs. The study of their development
shows, however, that these organs arise e^vCrgenously from
the node and have no root-cap : they are, in fact, not roots,
but submerged leaves, performing the function of roots.
XXXII
STRUCTURE
439
The latter organs are, quite exceptionally among the higher
plants, wholly absent.
The stem ends distally in a terminal bud (/. bd), the
f.l,3
Jil.1
sl.% sir
Fig. 1 1 4. — Distal portion of a Salvinia plant seen obliquely from
below.
The stem (j/) ends in a terminal bud (/. bd), and the part figured
contains three nodes, each bearing a pair of foliage-leaves (/ /. 1-3,
/. /. I'-s'), and a much-divided root-like submerged leaf (s. /. 1-3).
On the bases of the submerged leaves are borne groups of sori {so)t
containing sporangia. (Slightly enlarged.)
(From Vines, after Sachs.)
growing point of which is formed by a two-sided apical cell :
it is traversed by a single vascular bundle, which sends
branches into the leaves.
440
SALVINIA
LESS.
Springing from the bases of the submergetJ leaves are
numerous globular capsules (so), each containing a number
of sporangia. The wall of the capsule. (Fig. 115, a) corre-
sponds with the indusium of a fern, and the contained group
of sporangia with a sorus. But the sori of Salvinia, unlike
those of ordinary ferns, are dimorphic, some containing a
comparatively small number of large sporangia (mg. spg\
others a much larger number of small ones {mi, spg). The
larger kind, distinguished as megasporangia ^ contain each a
single large .spore, or megaspore : the smaller kind, or micro-
sporangia^ contain a large number of minute spores, like
those of an ordinary fern, and called microspores. It is this
striking dimorphism of the sori, sporangia, and spores which
forms the chief distinction between Salvinia and its allies
and the true ferns.
When ripe the sporangia become detached and float on
the surface of the water. The microspores germinate (b),
while still enclosed in their sporangium : each sends out a
filament, which protrudes through the wall of the micro-
sporangium, its extremity {spy) becoming separated off by a
septum and then divided into two cells. The protoplasm
of each of these divides into four sperm-mother-cells, and
from these spirally-twusted sperms are produced in the usual
manner. It is obvious that the two cells in which the
sperms are developed represent greatly simplified spermaries :
the single proximal cell {prth) of the filament arising from
the microspore, a still more simplified prothallus. Both
prothallus and spermaries are vestigial structures ; the pro^
thallus is microscopic and unicellular instead of being a
solid aggregate of considerable size, as in the two preceding
types ] each spermary forms only four sperm-mother-cells,
and the total number of sperms is therefore reduced to
eight.
xxxn REDUCTION OF THE GAMOBIUM 441
The contents of the megaspore are divisible into a com-
paratively small mass of protoplasm at one end, and of starch
grains, oil-globules, and proteid bodies, which fill up the rest
Fig. 1 15 — Reproduction and Development of Salvtma.
A, portion of a submerged leaf, showing three sori in vertical section,
two containing microsporangia {mi. spg) and one megasporangia {mg.
spg). (x 10.)
B, a germinating microspore {mi. spg)^ showing the vestigial prothallus
{prth) and its two spermaries {spy). { x .150.)
c, diagrammatic vertical section of a germinating megaspore, showing
the outer {mg. sp) and inner {mg. sp^) coats of the spore, and its cavity
{c) containing plastic products, separated by a septum {cl) from the pro-
thallus {prlA)f in which two ovaries {ovy) are shown, that to the left
containing an ovum, that to the right a polyplast. ( x 50. )
D, megaspore {mg. sp) with prothallus {prlA) and phyllula just begin-
ning to develop into the leafy plant : j/, stem ; r/, cotyledon ; and /,
outermost leaf of the terminal bud. ( x 20. )
(a and B, after Sachs ; D, after Pringsheim.)
(c, c) of the spore. The megasporc has, in fact, attained its
large size by the accumulation, of great quantities of plastic
products, which serve as nutriment to the future prothallus
442
SELAGINELLA
LESS.
and embryo, after the manner of the yolk in the eggs of the
crayfish and dogfish.
The protoplasm of the megaspore (c) divides and forms a
prothallus {prth) in the form of a three-sided multicellular
mass projecting from the spore, which it slightly exceeds in
size. Three ovaries (pvy) are formed on it, having much
the same structure as in ordinary ferns : if neither of these
should be fertilised others are developed subsequently.
'Fhus the reduction of the prothallus produced from the
megaspore, although obvious, is far less than in the case of
that arising from the microspore.
We see that sexual dimorphism has gone a step further in
Salvinia than in Equisetum : not only are the prothalli
differentiated into male and female, but also the spores from
which they arise.
Impregnation takes place in the usual way, and the
oosperm divides to form a polyplast, which, by differentiation
of a stem-rudiment, a cotyledon, and a foot, passes into the
phyllula stage : no root is developed in Salvinia. By the
gradual elongation of the stem (d, st) and the successive
formation of whorls of leaves (/), the adult form is assumed.
Thus the life-history of Salvinia resembles that of the
fern, but with two important differences : the spores are
dimorphic, and the gamobium, represented by the male and
female prothalli, is greatly reduced.
Selaginella
Selaginella, one of the club-mosses, is common on hill-
sides in many parts of the world. In the commoner species
there is a creeping stem which forks repeatedly in the hori-
zontal plane, and bears numerous small, close-set leaves,
giving the whole plant much the appearance of a moss.
XXXII
STRUCTURE
443
The leaves (Fig. ii6, a) arise in four longitudinal rows,
but, owing to the horizontal position of the plant, the two
rows belonging to the lower side (P) project laterally, and
Fig. ii6. — a, distal end of a shoot of Selaginella, showing the two
rows of small dorsal leaves (/'), the two laterally placed rows of ventral
leaves (Z^), and the terminal cone (c). (Nat. size.)
B, a microsporangium bursting to allow of the escape of the micro-
spores {mi. sp).
c, a megasporangium, with four megaspores {mg. sp).
(A, after Sachs ; B and c, after Le Maout and Decaisne.)
are many times larger than the two upper rows {P). Each
leaf bears on its upper or distal surface, near the base, a
small process called a ligule.
The stem usually ends in a two- or three-sided apical
cell, from which segments are cut off to form the apical
SELAGINELLA
LESS.
meristem, but in some species no apical cell can be distin-
guished. There are from one to three vascular bundles
running through the stem, each surrounded by a ring of
small air-cavities : from them a single bundle is given off to
each leaf. The presence of vascular bundles and of a well-
marked epidermis is enough to distinguish our present type
from the mosses, to which it bears a superficial resemblance.
The peculiar forked branching is due to the development
of lateral branches alternately on each side of the stem. The
roots arise from peculiar leafless branches, sometimes mis-
taken for true roots.
The branches terminate in cones (Fig. n6, a, c, and Fig.
1 1 7, a) formed of small leaves (jt/. pJi)^ which overlap in
somewhat the same way as the scales of a pine-cone. Each
of these leaves is a sporophyll, and bears on its upper or distal
side, near the base, a globular sporangium. The sporangia
are fairly uniform in size, but some are megasporangia
(Fig. 1 1 6, c, and Fig. 117, a, vig, spg)y and contain usually
four megaspores; others are microsporangia (Fig. 116, b,
and Fig. 117, a, mi\ spg), containing numerous microspores.
The microspore (Fig. 1 17, b) cannot be said to germinate at
all. Its protoplasm divides, forming a small cell (/rM), which
represents a vestigial prothallus, and a large cell, the repre-
sentative of a spermary.* The latter {spy) undergoes further
division, forming six to eight cells. in which numerous sperm-
mother-cells are developed. The sperms are finally liberated
by the rupture of the coats of the microspore.
A similar but less complete reduction of the prothallus is
seen in the case of the megaspore (c). Its contents are
divided, as in Salvinia, into a small mass of protoplasm at
one end,* and a large quantity of plastic products filling up
the rest of its cavity. The protoplasm divides and forms a
small prothallus {prth\ and a process of division also takes
Jfxxii PROTHALLUS 445
place in the remaining contents {prtk^) of the spore, pro-
ducing a large-celled tissue, the secondary prothallus.
By the rupture of the double cell-wall of the megaspore
Fig. 1 1 7 . — Reproduction and Development of Selaginelta.
A, diagrammatic vertical section of a cone, consisting of an axis bear-
ing close-set sporophylls (j/. ph)^ on the bases of which microsporangia
{mi. spg) and megasporangia {mg. spg) are borne.
B, section of a microspore, showing the outer coat {mi. sp)y prothallial
cell {prth)s and multicellular spermary {spy),
C, vertical section of a megaspore, the wall of which {mg. sp) has been
burst by the growth of the prothallus {prth ) : its cavity {prth^\ contains
a large-celled tissue, the secondary prothallus : in the prothallus are
three ovaries {ovy\ that to the left containing an ovum, that to the right
an embryo {emb) in the polyplast stage, and that in the centre an embryo
in the phyllula stage, showing stem-rudiment {st\ foot (/), and two
cotyledons {ct ) : both embryos are provided with suspensors (dotted)
{spsr)^ and have sunk into the secondary prothallus.
(Altered from Sachs. )
the prothallus is exposed to the air, but it never protrudes
through the opening thus made, and is, therefore, like the
corresponding male structure, purely endogenous. One or
446
SELAGINELLA
LESS. XXXII
more ovaries (pvy) are formed on it, each consisting of a
short neck, an ovum, and two canal-cells afterwards con-
verted into mucilage : there is no venter, and the neck con-
sists of only two tiers of cells.
The oosperm divides by a plane at right angles to the
neck of the ovary, forming the earliest or two-celled stage of
the polyplast. The upper cell undergoes further division,
forming an elongated structure, the suspensor (spsr ) : the
lower or embryo proper {emb) is forced downwards into the
secondary prothallus by the elongation of the suspensor,
and soon passes into the phyllula stage by the differentiation
of a stem-rudiment {st\ two cotyledons {ct\ a foot (/), and
subsequently of a root.
A further reduction of the gamobium is seen in Selagi-
nella : both male and female prothalli are quite vestigial,
never emerging from the spores : and the spermary and
ovary are greatly simplified in structure.
LESSON XXXIII
GYMNOSPERMS
The commonest Gymnosperms are the evergreen cone-
bearing trees such as pines, spruces, larches, cypresses, and
yews. They all have a primary axis or trunk from which
branches arise in a mpnopodial manner, the oldest are
near the proximal, the youngest near the distal end. The
branches give off, in successive seasons, branches of a higher
order, so that the older or lower branches are always them-
selves more or less extensively ramified, and the whole plant
tends to assume a conical form, the base of the cone being
formed by the oldest secondary axes springing from the
base of the trunk, the apex by the distal end of the primary
axis.
The branches are all axillary, each arising from the axil
of a leaf, and, like the main stem, ending distally in a
terminal bud. The foliage-leaves differ greatly in the various
genera of Gymnosperms : in the pines they are long, needle-
like structures, borne in pairs on short axillary branches or
dwarf-shoots.
In correspondence with the size attained by the aerial
portion of the plant, the root attains far greater relative
dimensions than in any case we have previously studied.
448
GYMNOSPERMS
LESS.
The trunk is continued downwards by a great primary root,
from which secondary roots arise in regular order, and, these
branching again and again, there is produced a root-system
of immense size and complexity, extending into the soil to a
sufficient depth to resist the strain to which the aerial part or
the tree is subjected by the wind.
One remarkable feature about the pines and their allies as
compared with the plants previously studied, is their practi-
cally unlimited growth. In mosses, ferns, &c., the stem
after attaining a certain diameter ceases to grow in thick-
ness, so that even in the tallest tree-ferns the stem is always
slender. But in pines the trunk, the branches, and the
roots continue to increase in thickness for an indefinite
period, the trunk in the common Scotch Fir {Pinus
sylvestris) attaining a circumference of four or five metres
or even more, and the other parts in proportion. The tree
may survive for hundreds of years.
The changes undergone during this remarkable process of
growth are best studied, in the first instance, by a series of
rough transverse sections of branches of different ages. In a
first year’s branch the middle is occupied by an axial strand
of soft tissue, the pith or medulla (f'ig. ii8, A and b, med ) ;
outside this comes a ring of wood {xy), divided into radially
arranged wedge-shaped masses ; and this in turn is sur-
rounded by the bark or cortex (cor), which can be readily
stripped off the wood, and which contains numerous resin-
canals (r. c) appearing in the section as rounded apertures
with drops of resin oozing from them. In a somewhat older
branch the layer of wood is seen to have increased greatly
in thickness, and has a well-marked concentric and radial
striation (c) : the cortex also has thickened though to a less
extent, while the pith is unaltered. The bark, moreover, is
clearly divisible into an inner light coloured layer, the bast
XXXIII
STRUCTURE OF STEM
449
or phloem {pkl)^ a middle green layer of cortical parenchyma
{cor) containing resin-canals, and an outer brown layer, the
cork {ck). Lastly, in the trunk and larger branches the wood
forms by far the greater part of the whole section, the bark
being a comparatively thin layer, easily stripped off, with no
JCfJt
Fig. ii 8. — Diagrammatic transverse sections of three branches of
Pinus of different ages.
A, very young axis, showing epidermis {ep)^ cortex {cor) with resin-
canals {r. c)y medulla {mccTjt and ring of vascular bundles, separated by
medullary rays {meet, r), and each consisting of xykm {xy)^ cambium
(cd), and phloem (pkl).
B, older axis, in which the cambium forms a complete cylinder, owing
to the formation of interfascicular cambium (cd') between the bundles.
c, Axis of the third year, showing xylem of first {xy^)^ second
{xy% and third {x}^) year’s growth ; cork {ck ) ; and cork -cambium
{ck. cb.)
cortical parenchyma, and with its corky outer layer much
thickened, gnarled, and wrinkled.
The wood has been stated to exhibit both concentric and
radial striations. The radial marking^ are called medullary
rays (Fig. ii8, c, med, r) and follow the “grain” of the
G G
450
GYMNOSPERMS
LESS.
wood. The concentric markings, which are against the
grain, are the annual rings xy\ and owe their
existence to the fact that the wood formed in summer and
autumn is denser than that formed in spring, while in winter
there is a cessation of wood-production. Thus, by counting
the annual rings of the main trunk, the age of the tree may
be estimated. The wood, it will be observed, grows from
within outwards, a new layer being added each year outside
the old.
The power of indefinite increase in diameter, which is so
striking a feature in the pines and their allies, is connected
with a peculiarity in the structure and arrangement of the
vascular bundles. In the very young condition, in the
terminal bud, the vascular bundles of the stem (Fig. ii8, a)
are wedge-shaped in transverse section and are arranged in
a circle, the apex of each being turned towards the axis of
the stem, the base towards its periphery. Actually, of course,
as in the fern, the bundles are longitudinal strands with pro-
longation into the leaves.
The arrangement of the tissues in the vascular bundles
differs in an important respect from the condition we are
familiar with in the fern. Instead of the xylem occupying
the centre of the bundle and being surrounded by phloem,
the xylem (Fig. ii8, a, xy) forms the whole of the in-turned
side, />., the narrow portion of the wedge in transverse sec-
tions, the phloem {pht) the outer portion or broad end of
the wedge. In a word, the bundles are not concentric as in
the fern, but collateral. Moreover, the phloem and xylem
are separated by a layer of small thin-walled cells, called the
cambium layer (cb\
By this arrangement of the vascular bundles the ground-
parenchyma of the stem is divisible into three portions, an
external layer, the cortex (cor), between the epidermis (ep),
XXXIII
GROWTH IN THICKNESS
451
and the phloem bundles, an axial cylinder, the pith or
medulla {med)y internal to the xylem bundles, and a series
of radial plates, the primary medullary rays {med, r) separat-
ing the bundles from one another.
As development proceeds the parenchyma-cells connecting
the cambium of adjacent bundles take on the characters of
cambium-cells, the result being the formation of a closed
cambium-cylinder, or, in transverse section, cambium-ring
(b, cby cb'). In this a distinction is to be drawn between
the fascicular cambium (cb) or original cambium of the
bundles and interfascicular cambium (cb') formed by con-
version of cells of the medullary rays.
The cambium-cells now begin to divide in a tangential
direction, i.e., along a plane parallel to the surface of the
stem. If this process went on alone the result would be
simply an increase in the thickness of the cambium layer,
but as it proceeds the products of division of the cells
on the inner face of the cambium-cylinder become con-
verted into new xylem-elements, those on its outer face
into new phloem-elements. We have thus a formation of
secondary tvood and secondary bast, which, being formed
from the whole of the cambium-cylinder, show no division
into bundles but form a continuous cylinder (c, xy, phi) of
constantly increasing thickness. The phloem now forms
the inner layer of the bark, which, as we have seen, can be
readily stripped from the wood owing to the delicate
cambium-cells being easily torn apart.
At the same time a layer of cells of the cortical parenchyma,
begins to divide tangentially so as to form a cylinder, or in
transverse section a ring, of cork-cambium (Fig. 118, c, ck,
cb)y from the outer face of which layer after layer of cork-
cells {ck) is formed. In the cork-cells the protoplasm dis-
appears and the cell-walls undergo a peculiar change by
G G 2
452
GYMNOSPERMS
LESS. XXXIII
which they become waterproof: this process, besides pro-
tecting the interior of the stem from external moisture,
prevents the access of nutrient matters to the epidermis
and outer layers of cortical parenchyma. Both these layers
consequently die and peel off, the outer surface coming to
be formed by the cork itself.
The wood of pines contains no vessels, cells joined end
to end so as to form a continuous tube, but only tracheides,
/.«?., elongated spindle-shaped cells with lignified walls and
devoid of protoplasm (p. 417). Radial bands of cells
mostly parenchymatous, are formed between the tracheides
of the secondary wood, and give rise to the secondary
medullary rays (c, med, r) to which the radial striation of
the wood is due : they increase in number with the increase
in thickness of the wood. The tracheides formed in
autumn have smaller cavities and thicker walls than those
formed in spring and summer: hence the formation of
annual rings. The tracheides are not scalariform like those
of ferns, but their walls have at intervals circular depressions
perforated in the centre and called bordered pits. The
tracheides of the primary xylem bundles have spirally
thickened walls, like the spiral vessels of ferns. The
phloem, both primary and secondary, consists of sieve-
tubes and parenchyma.
The growing point of Gymnosperms presents a striking
difference to that of ferns and other flowerless plants. It
consists simply of a mass of meristem cells among which no
apical cell is to be distinguished.
Pines, like horsetails and club-mosses, reproduce by
means of cones or flowers. These are of two kinds, ^male
and female, so that sexual differentiation is carried a step
further than in Selaginella, in which sporangia of both sexes
Fig. 1 1 9. — Reproduction and Development of Gymnos]^erms-
A, diagrammatic vertical section of male cone, showing axis with male
sporophylls {sp. ph. d ) bearing microsporangia (w/. spg ) : per^ scale-like
leaves forming a rudimentary perianth.
454
GYMNOSPERMS
LESS.
B, a single microspore, showing bladder-like processes of outer coat,
and contents divided into small prothallial cell {a) and large cell
from which the pollen-tube arises.
c, diagrammatic vertical section of female cone, showing axis with
female sporophylls {sp.ph, 9 ) bearing megasporangia {mg. spg), each of
which contains a single megaspore {mg. sp) : per^ the scale-like perianth
leaves.
1), diagrammatic vertical section of a megasporangium, showing
cellular coat (/), and nucellus («r/), micropyle {mp}f)y and megaspore
{mg, sp) : the latter contains the prothallus {prth) in which are two
ovaries, that to the left showing a large ovum {ov) and neck-cells, while
that to the right has given rise to an embryo {emb) which is in the
phyllula stage, and has sunk into the tissue of the prothallus by the
elongation of the long suspcnsor {spsr).
A microspore {mi. sp) is seen in the micropyle sending off a pollen-
tube (/. /), the end of which is applied to the necks of the two ovaries.
E, cfiagrammatic vertical section of a seed, showing coat (/), micro-
pyle {mpy)y and endosperm {end)y in which is imbedded an embryo in
the phyllula stage, consisting of stem-rudiment (j/), cotyledons (r/), and
root (r).
(a and B, altered from Strasburger ; D and E, altered from Sachs. )
are borne on the same cone. In the pines and their allies
both male and female cones are usually borne on the same
tree, so that the plant is monoecious : many Gymnosperms,
on the other hand, are dioecious^ each tree bearing either
male or female cones only.
The male cones (Fig, 119, a) arc borne in clusters or
inflorescences near the distal ends of the branches. Each
cone consists, as in Equisetum and Selaginella, of an axis
bearing a large number of sporophylls {sp. ph. ) : it springs
from the axil of a leaf and is to be looked upon as an
abbreviated and peculiarly modified shoot.
The sporophylls or stamens as they are commonly called
(Figs. 1 1 9, sp.ph. c? and Fig. 120), are more or less leaf-like
structures, each consisting of a short stalk or filament and an
expanded portion or anther^ the latter bearing on its under or
proximal side two microsporangia or pollen-sacs {mi. spg).
The mother-cells of these divide each into four microspores
or pollen-grains, which are liberated by the rupture of the
XXXIII
REPRODUCTIVE ORGANS
455
microsporangia in immense quantities, in the form of clouds
of light yellow powder called pollen. The microspore (b)
is at first an ordinary cell consisting of protoplasm with a
nucleus and a double cell-wall, but eventually the proto-
plasm divides into two cells ; a small one {a\ the vestige of
the male prothallus, which soon divides again forming two
or more cells, one of which is distinguished as the generative
cell \ and a large one {h\ the vegetative cell. Under favour-
able circumstances these cells undergo changes which will
be described presently.
The structure of the female cone is best made out in the
Fig. 120. — A single stamen or male sporophyll of the pine, showing
the two microsporangia or pollen-sacs.
larch. It also consists (Fig. 119, c) of an axis bearing
sporophylls {sp, ph, 9 ), or, as they are usually called in
Phanerogams, carpels. Each carpel is a crimson leaf with a
green midrib produced distally into a projecting point, and
bears on its upper or di.stal surface a little flattened body,
the placental scale^ on the upper surface of which are two
peculiarly modified megasporangia {mg. spg.)^ commonly
known as ovules. In the pine the placental scales (Fig. 121)
are larger than the carpels, and their thickened distal ends
form the rhomboid areas into which the surface of the cone
is divided.
4S6
GYMNOSPERMS
[.ESS.
The comparison of the reproductive organs of the pine
and larch with those of Vascular Cryptogams and of
Angiosperms will be facilitated by a consideration of two
exotic genera of palm-like Gymnosperms. In Zamia both
male (Fig. 122, a) and female (b) cones bear a close
external resemblance to those of Equisetum, the sporophylls
{sp, ph. S j sp. ph. $ ) being stalked hexagonal scales on the
inner surfaces of which the pollen-sacs (b, mi. spg) or ovules
(d, mg. spg) are borne. In the female Cycas the carpels
(e, sp. ph. 9 ) are not arranged in a cone, but form a whorl
Fig. 1 21. — A single carpel or female sporophyll of pine, with pla-
cental scale bearing two megasporangia or ovules.
of leaf-like bodies obviously homologous with foliage leaves.
Each carpel is, in fact, a leaf 20-30 cm. long, and deeply
lobed at its edge : in the distal portion the lobes are long
and slender, but proximally they take the form of ovoidal
bodies (mg. spg)^ about the size of plums, the ovules or
megasporangia.
The ovules differ strikingly in structure from the megaspor-
angia of Cryptogams. Each consists of a solid mass of small
cells called the nucellus (Fig. 119, d, nct)^ attached by* its
proximal end to the sporophyll, and surrounded by a wall
or integument (t) also formed of a small-celled tissue. The
XXXIII
reproductivp: organs
457
mgspg
Fig. 122. — A, male cone of Zamta, showing the hexagonal sporo-
phylls {sp. ph. $ ).
B, transverse section of the same, showing the microsporangia [mi,
spg) borne on the sporophylls.
C, distal end of female cone of Zamta^ showing the sporophylls [sp.
ph, 9).
D, transverse section of the same, showing the megasporangia [mg.
spg) borne on the sporophylls,
E, a single female sporophyll [sp. ph. 9 ) of Cycas, the pointed lobes
of the distal portion replaced proximally by megasporangia [?}ig. spg).
(After Sachs.)
integument is in close contact with the nucellus, but is per-
forsfted distally by an aperture, the micropyk {mpy\ through
which a small area of the nucellus is exposed.
Each megasporangium contains only a single megaspore,
GYMNOSPERMS
LESS.
.458
frequently called the embryo sac (c and d, mg-. sp\ and
having the form of a large ovoidal body embedded in the
tissue of the nucellus. It has at first the characters of a
single cell, but afterwards, by division of its nucleus and
protoplasm, becomes filled with small cells representing a
prothallus {prth). As in Vascular Cryptogams, single super-
ficial cells of the prothallus are converted into ovaries which
arc extremely simple in structure, each consisting of a large
ovum (pv\ and of a variable number of neck-cells.
The pollen, liberated by the rupture of the microsporangia,
is carried to considerable distances by the wind, some of it
falling on the female cones of the same or another tree. In
this way single microspores (pollen-grains) find their way
into the micropyle of a megasporangium (d, mi. sp). This
is the process known as pollination^ and is the necessary
antecedent of fertilisation.
The microspore now germinates : the outer coat bursts,
and the vegetative cell (b, b) protrudes in the form of a
filament resembling a hypha of Mucor, and called a pollen-
tube (d, /./). This forces its way into the tissue of the
nucellus, like a root making its way through the soil, and
finally reaches the megaspore in the immediate neighbour-
hood of an ovary. A process then grows out from the end
of the tube, passes between the neck-cells, and comes in
contact with the ovum.
In the meantime the nucleus of the vegetative cell (3) —
that from which the pollen-tube grows — has travelled towards
the end of the pollen-tube and undergone degeneration. The
generative cell at the same time enters the pollen-tube and
divides into two sperm-cells. The end of the pollen-tube
becomes mucilaginous arid one of the sperm-cells makes
its way through it, down the neck of the ovary and into the
ovum. The nucleus of the sperm-cell — called the male
XXXIII
FC RM ATION OF THE SEED
459
pronuckus — then conjugates with the nucleus of the ovum,
or female pronuckus^ and thus effects the process of fertilisa-
tion, or the conversion of the ovum into the oosperm.
The development of the oosperm is a very complicated
process, and results in the formation not of a single polyplast
but of four, each at the end of a long suspensor (n, spsr\
formed of a linear aggregate of cells, which by its elonga-
tion carries the embryo (cmlP^ down into the tissue of the
prothallus. As a rule only one of these embryos comes to
maturity : it develops a rudimentary stem, root, and four or
more cotyledons, and so becomes a phyllula.
While these processes are going on the female cone in-
creases greatly in size and becomes woody. The mega-
sporangia, now called seeds^ also become much larger, their
integuments (e, /), becoming brown and hard and constitut-
ing the seed-coat or testa^ which in the pine is produced into
a flattened expansion or wing. The megaspore in each seed
enlarges so much as to displace the nucellus : at the same
time the cells of the prothallus filling the megaspore develop
large quantities of plastic products, such as fat and albumin-
ous substances, to be used in the nutrition of the embryo :
the tissue thus formed is the endosperm {end).
As the cone dries the placental scales separate and expose
the seeds, which drop out and may be carried considerable
distances by the wind acting upon their wings, before falling
to the ground.
Under favourable circumstances the seed germinates.
By absorption of moisture its contents swell and burst the
seed-coat, and the root of the phyllula (r) emerges, followed
before long by the stem {st) and cotyledons (ct). The
phyllula thus becomes the seedling plant, and by further
growth and the successive formation of new parts is con-
verted into the adult.
46 o
GYMNOSPERMS
LESS. XXXIIl
In Gymnosperms we see an even more striking reduction
of the gamobium than in Selaginella. The female prothallus
is permanently inclosed in the megaspore, and the mega-
spore in the megasporangium : the ovaries also are greatly
simplified. The male prothallus is represented by the
smaller cell of the microspore, and no formation of sperms
takes place, fertilisation being effected by sperm-cells formed
from one of the products of division of the prothallial cell,
which migrate to the extremity of a tubular prolongation of
the larger or vegetative cell of the microspore, and finally
conjugate with the ova.
It is worthy of notice that in spite of the specialised
method of fertilisation in Phanerogams, the process is
essentially the same as in other organisms.
LESSON XXXIV
ANGIOSPERMS
To this group belong all the commoner herbs and shrubs
as well as trees other than Gymnosperms, such as palms,
oaks, elms, beeches, poplars, &c. There are two sub-
divisions of the group which must be mentioned, because
of the necessity of referring to them later on : they are the
Dicotyledons^ so called because of the presence of two coty-
ledons or seed-leaves in the phyllula, and the Monocotyledons^
in which only a single seed-leaf is present. AmongJDicp;^
tyledons are included the large majority of wild and garden
flowersV as well as most of the angiospermous trees : the
best known Monocotyledons are the lilies and their allies,
the various kinds of narcissus, orchids, grasses, and palms.
The general relations of the main parts of the plant —
stem, root, leaves, &c. — are the same as in Gymnosperms, as
may be seen by comparing a wallflower, an elm, a poplar,
and a lily, taken as examples of dicotyledonous herbs, of
dicotyledonous trees, and of Monocotyledons respectively.
In the lily, however, as in Monocotyledons generally, there
is no primary root, but a great number of equal-sized root-
fibres springing from the base of the stem.
In Dicotyledons the arrangement of the tissues is the
^ame as in Gymnosperms (p. 448) : the vascular bundles
462
ANGIOSPERMS
LESS.
are arranged in a circle, and there is a closed cambium
cylinder from which new xylem is added internally, and new
phloem externally. Moreover, in trees and shrubs,
plants which survive from year to year instead of dying down
at the end of one or two seasons, a cork-cambium is formed
Fig. 123. — Diagrammatic transverse section of the stem of a Lily,
showing the epidermis (<?/), cortical parenchyma containing chloro-
phyll {i or)t and axial cylinder of parenchyma surrounded by the pericycle
{pn) and containing vascular bundles, each consisting of phloem (pAl)
and xylem {xy),
in the cortex from which an external layer of cork is pro-
duced, the epidermis disappearing. So that the phenomena
of growth in thickness can be studied as conveniently in
any dicotyledonous tree as in a pine or cypress.
In Monocotyledons — in a lily, for instance — the arrange-
ment of tissues is different. The vascular bundles (Fig. 123)
are arranged in a number of irregular circles scattered
throughout the central parenchyma or ground tissue, which
XXXIV
VENATION
463
is separated from the cortical parenchyma (cort) by a layer
o( sclerenchymatous cells, the pericycle (J>rc). The bundles
are collateral, the xylem (xy) facing the axis of the stem,
the phloem (p/i/) its periphery : but there is a fundamental
difference from the bundles of Gymnosperms and Dico-
tyledons in that the fully formed bundle contains no
cambium, and is therefore incapable of further growth. The
bundles of Monocotyledons are therefore dosed, while those
of Gymnosperms and Dicotyledons are open. Owing
partly to this circumstance, partly to the thick unyielding
pericycle, the stems of nearly all Monocotyledons are in-
capable, when once their tissues are fully formed, of further
increase in thickness. Hence the* characteristic slenderness
of the trunk of a palm as compared with that of a pine or
an oak.
The wood of Angiosperms consists of spiral, annular
and dotted vessels, of fibres or prosenchymatous cells, and
of parenchyma. The phloem contains sieve-tubes, long
tough prosenchymatous cells called bast-fibres, and paren-
chyma. The growing point, as in Gymnosperms, has no
apical cell.
The leaves vary indefinitely in form, and all that* can be
mentioned with regard to them in the present brief sketch
is that in most Monocotyledons they are long and narrow,
and traversed by numerous parallel veins, while in Dico-
tyledons they are generally broad, with a smaller number —
one to five — of primary veins from which secondary veins
branch out and unite in a network. So that the venation
or veining is parallel in Monocotyledons, reticulate in Dico-
tyledons.
It is in the structure of the flower that the most striking
differences from, and the most marked advance upon,
464
ANGIOSPERMS
LESS.
Gymnosperms are seen. The modifications of the flower
among both groups of Angiosperms are almost infinite, and
can be thoroughly understood only by a careful study of
numerous forms : all that can be attempted here is to give
some idea of the essential points of structure and the lead-
ing modifications, by reference to a few selected forms.
In a buttercup {Ranunculus)^ one of the most generalised
Dicotyledons, the flower is borne at the end of a long stalk
or peduncle (Fig. 124, a and b, pd\ the distal end of which
is expanded into a conical floral receptacle (b and c, fl. r\
serving for the attachment of the various parts of the flower.
From the broad proximal end of the receptacle spring
five greenish leaves (a and b, cp\ arranged in a whorl : they
are the sepals^ and together constitute the calyx of the
flower. A little higher up arise, alternately with the sepals,
five larger leaves (a and b, pt) of a brilliant yellow colour,
forming the conspicuous part of the whole flower : they are
the petctls^ and together constitute the corolla. Each petal
has at the base of its upper side a little scale called a nectary
(f, nct\ from which a sweet juice, called nectar^ is secreted.
Both sepals and petals spring from the base of the conical
receptacle. From the lower half of the part above their
origin arise a large number of stamens (b and c, j/), arranged,
not in a whorl, but in a close spiral, and together constituting
the andrcecium. Each stamen (d) consists of a stalk or
filament {fl)^ bearing at its distal end an expanded body or
anther {an\ divided by longitudinal ridges into four lobes,
A transverse section (Fig. 125, b^) shows that each lobe
contains a pollen-sac or microsporangium {mi. spg\ filled, in
the ripe condition, with minute pollen-grains or microspores
{mi, sp).
From the distal portion of the receptacle arise, also in a
close spiral, a number of little pod-like bodies, the carpels
FLOWER
XXXIV
(b and c, cp) together constituting the gy?mciu?n or pistil.
Each carpel consists of an expanded, hollow, proximal
Fig. 124. — Structure of the flower of the Buttercup.
A, the entire flower from below, showing peduncle {pd)^ sepals {sp),
and petals (//).
B, vertical section of flower, showing peduncle {pd)^ floral receptacle
{Ji. r), sepals (sp), petals (//), stamen (j/), and carpels (^/).
The carpel cp' is cut vertically, and shows the megasporangiura.
C, floral receptacle {Jl. r), with carpels {cp), one stamen (i 7 ), and
scars left by the removal of the remaining stamens.
D, stamen, showing filament (y?) and anther {a??).
E, carpel in vertical section, showing venter {vn^) with contained
megasporangium sp^, and style (st).
F, petal, with nectary {net).
(a and c, after Vines ; B, D, and F, after Maout and Decaisne ; E,
after Oliver.)
portion or venter^ (e, vnt), and of a short, hook-like distal
extremity (st) covered with sticky hairs and called the stigma.
^ Commonly called ovary.
466 ANGIOSPERMS less.
The venter contains a single ovule or megasporangium {mg,
Sjfig)f differing from that of the pine in being covered by a
double instead of a single coat (Fig. 126, D, both
perforated by a micropyle (m, py\ which places the central
mass of tissue or nucellus {net) in communication with the
cavity of the venter (Fig. 126, a). The nucellus, like that
of pines, contains a single embryo-sac or megaspore {nig, sp).
The fact that the megasporangia are contained in a cavity
of the carpel, and so shut off from all direct communication
with the exterior, forms a fundamental difference between
the angiospermous or covered-seeded, and the gymno-
spermous or naked-seeded Phanerogams.
We saw that in Gymnosperms, as in the Vascular Crypto-
gams, the sporangia were borne on structures, the sporophylls,
which were obviously modified leaves. In the buttercup
the stamens and carpels have departed so widely from the
leaf-type that their true nature becomes obvious only after
comparison with other forms.
In the White Water-lily {Nymphaa alba) the very numerous
petals are arranged, like the stamens, in a spiral, and the
two sets of organs pass insensibly into one another. As we
trace the petals (Fig. 125, a^) upwards on the floral recep-
tacle we find them become narrower in proportion to their
breadth (a®), and on the apex two little yellow lobes appear
{mi, sp^. Still passing up the spiral the lobes become
more and more pronounced, and the petal narrower (a®),
until at last the lobes become aggregated into an undoubted
anther (a^, an)^ while the blade of the petal is narrowed to a
filament, its distal end serving to unite the anther-lobes and
constituting the connective {cor).
The same transition from petals to stamens is seen in
many “ double ” flowers, such as the double apple, in which
the number of petals becomes greatly increased by the
XXXIV
MORPHOLOGY OF FLOWER
467
assumption of a petaloid form by the outer stamens, various
intermediate stages being present from the typical stamen,
through irregular leaves with anther-lobes at their distal ends,
to the ordinary broad white petal.
We see, then, that a stamen is a leaf on the surface ot
which four microsporangia (b^, mi, spg) are developed ; the
blade of the leaf is narrowed to form a mere stalk, while the
Fig. 125.— A^-A^, transition from petal to stamen : mi. si>g^ micro-
sporangia ; filament ; anther.
transverse section of male sporophyll in the stage A® ; wr, mid-
rib of staminal leaf ; mi. spg, microsporangia.
B®, transverse section of typical anther, showing connective (cor) with
vascular bundle or midrib (wr), on the left two microsporangia (mi.
spg), and on the right the escape of the microspores (mi. sp) by dehis-
cence of the anther.
microsporangia have become so closely aggregated as to
form a single four-lobed body, the anther (b®).
Similarly the carpel can be shown to conform to the leaf-
type. The flower of the cherry has a single flask-shaped
carpel, consisting of a rounded venter, with an expanded
stigma borne on the end of a stalk or style. But when the
cherry flower becomes double, the normal carpel is replaced
by a little green leaf, quite like a foliage-leaf, except that it
is • permanently folded upon the midrib so as to bring the
two halves of its upper or dorsal surface almost into contact.
H H 2
468
ANGIOSPERMS
LESS.
Imagine one or more of the marginal lobes of such a leaf to
be replaced by megasporangia, as in Cycas (Fig. 122, e ),
and the edges of its proximal part to come together and
unite (Fig. 126, b\ b^). The result will be the enclosure
of the ovules in a capsule formed from the proximal part of
the leaf, while its distal end forms the style and stigma.
The extreme differentiation of both male and female
sporophylls is not the only important difference between the
angiospermous and the gymnospermous flower. Almost
equally characteristic, and equally striking as a sign of
advance in organisation, is the fact that the sporophylls are
surrounded by two sets — ^sometimes reduced to one— of
floral organs, the sepals and petals, which together form the
floral envelope or perianth. In most Gymnosperms the
only indication of a perianth is in the form of inconspicuous
oarren scales, scales not bearing sporangia, at the base
of the cone ‘(Fig. 119, a and B,/^r), while in Angiosperms
the perianth has become differentiated into two well-marked
and conspicuous sets of leaves.
The function of the sepals is usually to protect the other
parts of the flower in the bud : they are generally of such a
size as completely to close over the petals, stamens, and
carpels until the flower opens, when they often either turn
back or fall off. They are therefore to be looked upon as
leaves which have been modified for protective purposes.
The petals serve an entirely different function. They are
usually large and brightly coloured, forming the most con-
spicuous part of the flower : they are also commonly scented,
and from them or some adjacent part nectar is secreted.
This fluid forms the staple food of many insects, especially
butterflies, moths, and bees, which, as soon as a flower is
opened, may be seen to visit it and to insert head or
proboscis in order to suck the sweet juice.
XXXI V
MORPHOLOGY OF FLOWER
469
By the time this takes place the stamens have dehisced^
split down each side, so that the two pollen-sacs of each
half-anther discharge their pollen by a common slit (Fig. 125,
B^). The pollen is usually not dry like that of Gymnosperms
but sticky, so that the grains are not readily blown away but
tend to adhere to one another and to the ruptured anther.
Thus, when the insect inserts its head into the flower a
greater or less quantity of pollen is certain to stick to
it, and to be carried off as the insect flies to another
flower.
It will be remembered that the stigma is cohered with
sticky hairs, the consequence of which is that as the insect
flies from flower to flower, the pollen it has collected from
the stamens of one is transferred to the stigmas of another,
and thus, in all the higher Angiosperms, pollination is effected
by the agency of insects and not, as in Gymnosperms, by the
chance action of the wind.
Thus the corolla serves an attractive purpose: by its
colour and scent insects are informed of the store of nectar
it contains, and in the search for that food they uncon-
sciously benefit the plant by performing the work of pollina-
tion. In this way pollination is made more certain than
when left to the wind, and the plant is saved the production
of the immense quantity of pollen essential to a wind-
fertilised plant, in which a very small fraction of the grains
produced can possibly find their way to a female cone.
Still another striking feature of the angiospermous as
compared with the gymnospermous flower is the shortening
of its axis. A comparison of Fig. 126, a, with Fig. 119,
A and ,c, shows that the floral receptacle {ft. of the Angio-
sperm is nothing but the axis of the gymnospermous con^
shortened and broadened. The natural result is the suppres-
sion of the internodes and the consequent approximation
Fir.. 121). - Ivrpio.luciiun and Uevelopmeui oT Angiosperms.
A, diagrammatic vertical section of a flower consisting of an abbreviated
axis or floral receptacle { fl. r) bearing a proximal {per^) and a distal
{per^) whorl of perianth leaves (sepals and petals), a whorl of male
sporophylls or stamens {sp, ph. d ), and one of female sporophylls or
carpeis {sp. pk, 9 )•
LESS. XXXIV
MORPHOLOGY OF FLOWER
471
The male sporophyll bears microsporangia (mi. spg) containing
microspores [mi. sp).
The female sporophyll consists of a solid style (si) terminated by a
stigma and of a hollow venter (v) containing a megasporangium
(m£. spg) in which is a single megaspore (mg. sp).
On the right side a microspore is shown on the stigma, and has sent
off a pollen-tube (p.f) through the tissue of the style to the micropyle of
the megasporangium.
B^, diagram of a female sporophyll from the dorsal aspect, and B*, the
same in transverse section, showing the folding in of its edges to form
the cavity or venter in which the megasporangia (mg. sp^) are enclosed :
m. r, the midrib.
c^, a microspore, showing the two cells (a and b) into which its
contents divide ; the larger is the vegetative-cell.
c\ the same, sending out a pollen-tiibe (p. t)\ nu^ the two nuclei :
the generative nucleus has not yet divided.
D, diagrammatic vertical section of a megasporangium, showing the
double integument (/^,/*), nucellus («r/), micropyle (m.py)^ and mega-
spore (mg. sp ) ; the latter contains the secondary nucleus (;///) in the
centre, three antipodal cells (ant) at the proximal end, and two syner-
gidae (sng) and an ovum (ov) at the distal end.
A pollen-tube (/. t) is shown with its end in contact with the
synergidae.
E, semi-diagrammatic section of the megaspore of a young seed,
showing an embryo (emb) in the polyplast stage with its suspensor
(spsr) ; also numerous vacuoles (va^ and nuclei (mi).
F, diagrammatic vertical section of a ripe seed, showing the seed-coat
(/), micropyle (w. py)^ perisperm (per) derived from the tissue of the
nucellus, and endosperm (end) formed in the megaspore and containing
an embryo in the phyllula stage with stem-rudiment (j/), cotyledons (ct)^
and root (r).
(b\ after, Behrens ; c^, c'^, and E, altered from Howes.)
of the nodes, so that all the leaves — sepals, petals,
stamens, and carpels — arise close together from a small
area. Thus, the angiospermous flower, like the gymno-
spermous cone, is a modified shoot of limited growth,
having its axis shortened to a floral receptacle and its
leaves modified to form the various floral organs. The
composition of the flower may therefore be expressed in a
diagrammatic form as follows : —
( Pi>rinnfTi / Protcctive-— Scpuls (Calyx).
Floral Receptacle \ Attractive — Petals (Corolla),
sr Axis of Shoot / j Sporo- /Male — Stamens (Andrceciuin).
^ phylls \ Female — Carpels (Gynoccium).
472
ANGIOSPERMS
l,ESS,
vnt
Fig. 127. — A^, Vertical section of flower of Helleborus^ showing^?,
floral receptacle ; sp, sepals ; pt^ petals ; st^ stamens ; and r/, carpels,
that to the right cut longitudinally to show the megasporangia {ntg. spg).
transverse section of gynoecium of Helleborus passing through the
venter of the six, carpels, each of which has a midrib {im) and
united edges (e) to which t^he megasporangia are attached.
vertical section of flower of Campanula^ showing floral rece^-
tacle {fi. r) enclosing venter of gynoecium {vnt)^ with megasporangia
[mg, spg) ; calyx (ca^ ; corolla {cor) ; anthers {an) and filaments (yf) of
stamens ; and style {sty) and stigma {stg).
B®, transverse section of gynoecium of Campanula enclosed in floral
receptable (/?. r). Letters asdn A®.
c, transverse section of gynoecium of Ribes, Lette^rs as in A^.
(a^ and B^, after Le Maout and Decaisne.)
XXXIV
MODIFICATIONS OF FLOWER
473
There are one or two important modifications of the
flower which must be briefly referred to.
In the Christmas-rose {Helleborus) the general structure
of the flower resembles that of the buttercup except that the
petals (Fig. 127, a^, pi) are small and tubular, and the
sepals (sp) so large as to form the obvious and attractive
part of the flower. But the large carpels (cp) are few — three
to six — in number, arranged in a single whorl, and closely
applied to one another by their lateral faces (a^). The
peripheral or outwardly-facing border of each represents the
midrib (mr) of the carpellary leaf, the central border — that
facing the axis of the flower — its united edges (e). To the
-latter are attached several megasporangia arranged in a
longitudinal row.
In the Canterbury-bell ( Campanula) there appears at first
sight to be a single carpel (b^ vni) with three stigmas {st^.
But a transverse section of the venter (b^) show's it to
contain three cavities arranged round a longitudinal axis to
which are attached three rows of ovules (mg. sp^^ one to
each chamber. Obviously such a pistil is produced by the
three carpels of which it is composed being not simply
applied to one another as in the Christmas-rose, but actually
fused. In the currant (Ribes) the pistil shows in transverse
section a single cavity only (c), but with two rows of ovules
(mg. spg ) : here the carpellary leaves have united with one
another simply by their edges.
Campanula illustrates concrescence not of the carpels
only but of all the other floral whorls. The sepals have
united to form a cup-like calyx (Fig. 127, cal)^ the petals
are joined into a vase-like corolla (cor), and the filaments* of
the stamens (fi) are united below. Moreover, the floral
receptacle (fl. r) instead of being conical, as in the butter-
cup, is hollowed into a cup which encloses and is fused with
474
ANGIOSPERMS
LESS.
the venter of the pistil (^nt) : it thus loses all appearance of
being a stem-structure and becomes a mere capsule for the
gyncecium, giving attachment at its edges to the other
floral organs.
An extended study of flowers will show how all the main
modifications are brought about by the varying form of
the floral receptacle, by the concrescence of one part with
another, by the enlargement of certain parts, and by the
diminution or complete suppression of others.
The microspores are at first simple cells, each with a
double cell-wall and a nucleus. The nucleus divides into
two (Fig. 126, c^), a larger vegetative nucleus, and a smaller
which divides again forming two generative nuclei, each
surrounded by its layer of protoplasm.
No prothallus is formed in the megaspore, but its nucleus
divides, the products of division pass to opposite ends of
the spore, and each divides again and then again, so that
four nuclei are produced at each extremity. Three of the
nuclei at the proximal end — that furthest from the micropyle
— become surrounded by protoplasm and take on the char-
acter of cells (d, ani) all devoid of cell- wall and called antipodal
cells ; the fourth remains unchanged. Similarly, of the four
nuclei at the distal or micropylar end, one remains unchanged
and three assume the form of cells by becoming invested
with protoplasm. Of these three, two lie near the wall of
the megaspore and are called synergidee {sng)\ the third, more
deeply placed, is the ovum {ov). The two unaltered nuclei
now travel to the centre of the megaspore and unite, with
one another, forming the secondary nucleus (nu) of the spore.
There is thus a single ovum produced in each megaspore,
but no ovary and no prothallus : the female portion of the
gamobium is reduced to its simplest expression.
XXXIV
POLLINATION AND FERTILISATION
475
Pollination may take place, as we have seen, by the agency
either of the wind or of insects. The microspores are
deposited on the stigma (a), where they germinate, each
sending off a pollen-tube (a and c^, /), which growls
downwards through the tissue of the stigma and style to the
cavity of the venter, where it reaches a megasporangium,
and entering at the micropyle (d, /. /), continues its course
through the nucellus, finally applying itself to the distal end
of the megaspore in the immediate neighbourhood of the
synergidae.
In the meantime the nuclei of the microspore (c^, nUy
nu^) have passed into the end of the pollen-tube. The
vegetative nucleus undergoes degeneration, becoming
shrivelled and unaffected by dyes. The generative nuclei
wander to the apex of the pollen-tube and ultimately pass
through the softened cell-wall of its swollen end, one of them
entering the ovum and uniting with its nucleus in the usual
way, while the other fuses with the secondary nucleus of the
megaspores.
The ovum is thus converted into an oosperm or unicellu-
lar embryo : it acquires a cell-wall and almost immediately
divides into two cells, of which that nearest the micropyle
becomes the suspensor (e, spsr), the other, or embryo
proper {emb), forming , a solid aggregate of cells, the poly-
plast. By further differentiation rudiments of a stem (f, st\
a root {r) and either one or two cotyledons (ct) are formed,
and the embryo passes into the phyllula stage.
While the early development of the embryo is going on,
the secondary nucleus of the megaspore divides repeatedly,
and the products of division (e, nu) becoming surrounded
by protoplasm, a number of cells are produced, which, by
further multiplication, fill up all that part of the megaspore
which is not occupied by the embryo. The tissue thus
476
ANGIOSPERMS
LESS. XXXIV
formed is called the endosperm (f, end)^ and occupies pre-
cisely the position of the vestigial prothallus of Gymnosperms
(Fig. 119, p. 453, and e, end: and p. 458), differing
from it in the fact that it is formed only after fertilisation.
We have here a case of retarded development : the degenera-
tion of the prothallus has gone so far that it arises long
after the formation of the ovum which, in both Gymnosperms
and Vascular Cryptogams, is a specially modified prothallial
cell. As a rule the tissue of the nucellus disappears as the
embryo grows, but in some cases, the water-lily, it is
retained, forming an additional, store of nutrient material
and called the perisperm (Fig. 126, f, per\
The phyllula continues to grow and remains inclosed in
the megasporangium, which undergoes a corresponding in-
crease in size and becomes the seed. One or more seeds
also remain inclosed in the venter of the pistil, which grows
considerably and constitutes the fruit. Finally the seeds
are liberated, the phyllula protrudes first its root, and then
its stem and cotyledons, through the ruptured seed-coat, and
becomes the seedling plant.
We learn from this and the two preceding lessons that
there is a far greater uniformity of organisation among the
higher plants than among the higher animals, not only in
anatomical and histological structure, but also in the fact
that alternation of generations is universal from mosses up
to the highest flowering plants. But as we ascend the
series, the gamobium sinks from the position of a conspicu-
ous leafy plant to that of a small and insignificant prothallus,
becoming finally so reduced as to be recognisable as such
only by comparison with the lower forms.
SYNOPSIS
A.— AN ACCOUNT OF THE STRUCTURE, PHYSIOLOGY.
AND LIFE-HISTORY OF A SERIES OF TYPICAL
ORGANISMS IN THE ORDER OF INCREASING
COMPLEXITY.
I.— The Simpler Unicellular Organisms.
I Amosba,
vAim
Cell-body amoeboid or encysted : cell- wall nitrogenous (?):
nutrition holozoic : reproduction by simple or binary
fission I
2 . Hamatococcus,
Cell-body ciliated or encysted : cell-wall of cellulose :
nutrition holophytic : reproduction by binar^ fission . . 23
3, Heteromita,
Cell-body ciliated ; nutrition saprophytic : asexual repro-
duction by binary fission : sexual reproduction by conju-
gation of equal and similar gametes followed by multiple
fission of the protoplasm of the zygote, forming spores . 36
4. Euglena,
Cell-body ciliated or encysted : cell-wall of cellulose :
mouth and gullet present : nutrition holophytic and
holozoic : reproduction by binary and multiple fission . . 44
5, Protomyxa,
Cell-body amoeboid, ciliated, or encysted; plasmodia
formed by concrescence of amoebulae; cell-wall nitro-
genous (?) ; nutrition holozoic : reproduction by multiple
fission of encysted plasmodium
49
478
SYNOPSIS
PACE
6. Mycetozoa,
Like Protomyxa, but owing to the presence of nuclei the
relatioii of the individual cell -bodies to the plasmodium
is more clearly seen ; cell-wall of cellulose 52
7. Saccharomyces.
Cell-body encysted : cell-wall of cellulose : nutrition
saprophytic ; reproduction by gemmation or by internal
fission ; acts as an organised ferment 71
8. Bacteri a,
'Cell body ciliated or encysted : cell -wall of cellulose :
nutrition saprophytic : reproduction by binary fission or
by spore-formation : act as organised ferments : the
simplest and most abundant of organisms 82
II, — Unicellular or Non-cellular Organisms in which there
IS considerable Complexity of Structure accompanied
BY Physiological Differentiation.
a. Complexity attained by differentiation of cell-body,
9. Paratnarium,
Medulla, cortex, and cuticle : trichocysts : complex
contractile vacuoles : mega- and micro-nucleus : mouth,
gullet, and anal spot : conjugation temporary, no zygote
being formed, but interchange of nuclear material during
temporary union 106
10. Stylonychia.
Extreme differenfiation or heteromorphism of cilia ... 116
II. Oxytricha,
Fragmentation of nucleus
120
12. Opalina,
Multinucleate but non-cellular: parasitism and its results :
necessity for special means of dispersal of an internal
parasite 12 1
13. Vorticella.
*A stationary organism : limitation of cilia to defined
regions : muscle-fibre in stalk : necessity for means of
dispersal in a fixed organism : conjugation between free-
swimming micro- and fixed mega-gamete : zygote indis-
tinguishable from a zooid of the ordinary kind 126
SYNOPSIS
479
FAGB
14. Zoothamnium.
A compound organism or colony with dimorphic (nutri-
tive and reproductive) zooids : logins life as a single
zooid 134
b. Complexity attained by differentiation of cell-wall or by forma-
tion of skeletal structures in the protoplasm,
15. Foraminifera,
Calcareous shells (cell-walls) of various and complicated
form 148
16. Radiolaria.
Membranous perforated shell (cell-wall) and external
silicious skeleton often of great complexity : symbiotic
relations with Zooxanthella 1 52
17. Diatoms,
Silicious, two valved, highly-ornamented shells ... 155
Complexity attained by simple elongation and branching of the
cell.
18. Mucor,
A branching filamentous non-cellular fungus ; necessity
for special reproductive organs in such an organism : they
may be sporangia producing asexual spores, or equal and
similar gametes producing a resting zygote 158
19. Vaucheria,
A branched filamentous non-cellular alga : clear distinc-
tion between the gametes or conjugating bodies and the
sexual reproductive organs or gonads in which they are
produced : gonads differentiated into male (spermary)
and female ^ovary) : gametes differentiated into male
(sperm) and female (ovum) : zygote an oosperm .... 169
20. Caulerpa,
Illustrates maximum differentiation of a non-cellular
plant : stem-like, leaf-like, and root -like parts 174
III.— Organisms in which Complexity is attained by Cell-
Multiplication, ACCOMPANIED BY NO OR BUT LITTLE CELL-
D IFFERENTI ATION.
a. Linear aggregates.
21. Penicillium.
A multicellular, filamentous, branched fungus : mycelial,
submerged, and aeriql hyphae : apical growth : abundant
production «of spores by constiiction of aerial hyphse . . 184
480
SYNOPSIS
PAGE
22. Agaricus.
Complexity attained by interweaving of hyphae in a de-
finite form : illustrates maximum complexity of a linear
aggregate 19 *
23. Spirogyra.
A multicellular filamentous unbranched alga : interstitial
growth : gonads equal and similar, but gametes show
first indications of sexual differentiation 194
b. Superficial aggregate.
24. Monostroma.
Cell-division takes place in two dimensions 201
c. Solid aggi'egate.
25. Ulva.
Like Monostroma, but cell-division taxes place in three
dimensions 203
IV.— Solid Aggregates in which Complexity is increased
BY A limited amount OK Cell-Differrntiation.
26. Nitella.
Segmented axis : nodes and intemodes : appendages —
leaves and rhizoids : apical growth by binary fission of
apical cell accompanied by immediate division and dif-
ferentiation of newly-formed segmental cells; complex
gonads (ovaries and spermaries) 203
27. Hydra.
Example of a simple diploblastic animal : cells arranged
in two layers (eclo- and endoderm) enclosing an enteron
which opens externally by the mouth : combination of
intra-cellular with extra-cellular or enteric digestion . . 218
28. Bougainvillea.
Example of a colony with diploblastic zooids which are
nutritive (hydranths) and reproductive (medusae) ; differ-
entiation of a rudimentary mesoderm producing imper-
fect tripoblastic condition ; central and peripheral nervous
system : alternation of generations, a gamobium (the
medusa) alternating with an agamobium (the hydroid
colony) ; significance of developmental stages— oosperm
(unicellular), polyplast (multicellular but undifferenti-
ated), and planula (diploblastic) 234
SYNOPSIS
481
PAGE
29. Diphyes, .
A free-swimming colony with polymorphic (nutritive,
reproductive, protective, and natatory) zooids 248
30. Porpita,
Extreme polymorphism of zooids giving the colony the
character of a single physiological individual 249
V. — Solid Aggregates in which Cell-Differentiation, ac-
companied BY Cell-Fusion, takes an important part in
producing great Complexity in the Adult Organism.
31. Polygorditts,
A triploblastic, ccelomate animal with metameric seg-
mentation : prostomium, peristomium, metameres, and
anal segment : besides ecto- and endoderm there is a
well developed mesoderm divided into somatic and
splanchnic layers separated by the coelome : differenti-
ation of cells into fibres, &c. : muscle-plates formed aS
cell-fusions : necessity for distributing system for supply
of food to parts of the body other than the enteric canal,
and for the removal of waste matters : — circulatory,
respiratory, and excretory systems : high development of
nervous system — brain and ventral cord, afferent and
efferent nerves : characteristic developmental stages —
oosperm, polyplast, gastrula (diploblastic), trochosphere
(diploblasfic with stomodseum and protodseum), late
trochosphere (triploblastic but acoelomate) 268
32. Mosses.
Cell-differentiation very slight, but the t^pe necessary to
lead up to ferns : sclerenchyma and axial bundle : dis-
tributing system rendered necessary by carbon dioxide
being taken in by the leaves, water and mineral salts by
the rhizoids : alternation of generations — the leafy plant
is the gamobium, the agamobium being represented by
the spore-producing sporogonium : developmental stages
— oosperm and polyplast, the latter becoming highly
differentiated to form the sporogonium 400
33. Ferns.
Extensive cell -differentiation : formation of fibres (elon-
gated cells) and vessels (cell-fusions) ; general differenti-
ation of tissues into epidermis, ground- parenchyma, and
vascular bundles : presence of true roots : the leaf^ plant
is the agamobium and produces spores from which the
gamobium, in the form of a small prothallus, arises :
developmental stages — oosperm, polyplast, and phyllula
(leaf- atid root-bearing stage) 412
1 1
482
SYNOPSIS
PAOlt
VI.— Brief Descriptions of Examples of the Higher Groups
OF Animals and Plants.
a. ANIMALS,
All are triploblastic and coelomatc.
34. Starfish,
Radially symmetrical : discontinuous dermal exoskele-
ton : characteristic organs of locomotion (tube feet) in
connection with ambulacral system of vessels 306
35. Crayfish,
Metamerically segmented: segmented lateral append-
ages : differentiation of metameres and appendages :
continuous cuticular exoskeleton discontinuouSly calci-
fied : gills as paired lateral offshoots of the body-wall :
heart as muscular dilation of dorsal vessel ; ccelome
greatly reduced and its place taken by an extensive series
of blood -spaces : nervous system sunk in the mesoderm
and consisting of brain and ventral nerve-cord 318
36. Mussel.
Non-segmented : mantle formed as paired lateral out-
growths of dorsal region : foot as unpaired median out-
growth of ventral region ; cuticular exoskeleton in the
form of a calcified bivalved shell : gills as paired lateral
outgrowths of body- wall : heart as muscular dilatation of
dorsal vessel : coelome reduced to pericardium : nervous
system consists of three pairs of ganglia sunk in the
mesoderm 348
37. Do\:fish,
Metamerically segmented : differentiated into head, trunk,
and tail : trunk alone ccelomate in adult : appendages as
median (dorsal, ventral, and caudal) and paired (pectoral
and pelvic) fins ; discontinuous dermal exoskeleton and
extensive endoskeleton of partially calcified cartilage,
including a chain of vertebral centra below the nervous
system replacing an embryonic notochord : gills as
pouches of pharynx opening on exterior : heart as
muscular dilatation of ventral vessel : hollow dorsal
nervous system not perforated by enteric canal 366
b, PLANTS,
All exhibit alternation of generations and the series
shows the gradual subordination of the gamobium to the
agamobium.
SYNOPSIS
483
PAGE
38. Eqitisetum.
Sporangia home on sporophylls arranged in cones :
spores homomorphic : prothalli dimorphic (male and
female) 434
39. Saivinia.
Spores dimorphic : microspore produces vestigial male
prothallus : megaspore produces greatly reduced female
prothallus 438
40. Selaginella.
Microspore produces unicellular prothallus and multi-
cellular spermary, both endogenously : female prothallus
formed in megaspore and is almost endogenous ; embryo
provided with suspensor 442
41. Gymnosperms.
Cones dimorphic (male and female), with rudimentary
perianth : no sperms formed but microspore gives rise to
pollen tube, nuclei in which are the active agents in fer-
tilisation : single megaspore permanently enclosed in each
megasporangium : female prothallus purely endogenous :
embryo (phyllula) remains enclosed in megasporangium
which becomes a seed 447
42. Angiosperms.
Cone modified into flower by differentiation of sporo-
phylls and ]3erianth ; female sporophyll forms closed
cavity in which megasporangia are contained : mega-
spore produces a single ovary rejnesented simply by an
ovum and two synergida;: foimation of prothallus re-
tarded until after fertilisation 461
B.— SUBJECTS OF GENERAL IMPORTANCE DISCUSSED
IN SPECIAL LESSONS.
1.— Cells and Nuclei,
a. The higher plants and animals contain cells similar in struc-
ture to entire unicellular organisms, and like them exist-
ing in either the amoeboid, ciliated, encysted, or plas-
'modial condition 56
b. Minute structure of cells : — cell-protoplasm, cell -membrane,
nuclear membrane, nuclear sap, chromatin 62
r. Direct and indirect nuclear division 6^
I I 2
484 SYNOPSIS
PAGE
d. The higher plants and animals begin life as a single cell, the
ovum 68
II. — Biogenesis.
a. Definition of biojjenesis and abiogenesis : brief history of
the controversy 95
b. Crucial experiment with putrescible infusions : sterilisation :
germ -niters : occurrence of abiogenesis disproved under
known existing conditions 98
III. —Homogenesis.
Definition of homogenesis and heterogenesis ; truth of the
former firmly established 102
IV.— Origin of Species.
a. Meaning of the term Species : the question illustrated by a
consideration of certain species of Zoothamnium . ... 137
b. Definition of Creation and Evolution : hypothetical histories
of Zoothamnium in accordance with the two theories . . 141
t\ The principles of Classification : natural and artificial
classifications 141
d. The connection between ontogeny and phylogeny 146
V.-— Plants and Animals.
a. Attempt to define the words plant and animal, and to place
the previously considered- types in one or other king-
dom 176
A Significance of “third kingdom,” Protista 182
VI. — Spermatogenesis and Oogenesis.
Origin of sperms and ova from primitive sex-cells : differences
in structure and development of the sexual elements . . 253
VII. — Maturition and Impregnation.
a, formation' of first and second polar cells and of female
pronucleus 257
b Entrance of sperm and formation of male pronucleus . . . 260
w. Conjugation of pronuclei 260
SYNOPSIS
48s
PAGE
VIIT.— UNICFXLUI.AR AND DiPLOBLASTIC ANIMALS.
In plants there is a clear transition from unicellular forms to
solid aggregates, but in animals the connection of the
gastrula with unicellular forms is uncertain 261
C. —'Other matters of general importance, such as the composition
and properties of protoplasm, cellulose, chlorophyll, starch, &c. : meta-
bolism : holozoic, holophytic, and saprophytic nutrition : intra- and
extra-cellular digestion : amoeboid, ciliary, and muscular movements :
the elementary physiology of muscle and nerve : parasitism and sym-
biosis : asexual and sexual generation ; and the elements of embryology
— are discussed under the various types, and will be most conveniently
referred to by consulting the Index.
INDEX AND GLOSSARY
INDEX AND GLOSSARY
A.bdoineii, Crayfish, 318
Abio^teil'esla (a, not: /Sios, life: yeVe<n?,
origin;, tne origin of organisms from
not-Iiving matter : former belief in, g6
AbSOrptioiL by toot-hairs, 409, 422
Aocre^tlon (arf, to : cresco^ to grow), in-
crease by addition of successive layers,
14
Adhrom^atin (a, not : colour), the
constituent' of the nucleus which is un-
affected or but slightly affected by dyes.
See nuclear .sap
AocBlom'ate (a, not : icoiAco^ta, a hollow),
having no coelome : 299
AddUOt'or muscles, Mussel, 354
Agr2&i& i^VP} ^ii* • iif^A applied to
those microbes to which free oxygen is
unnecessary, 93
A yanobllim (a, not : yaftot, marriage :
ptos, me], tne asexual generation in or-
ganisms exhibiting alternation of gene-
rations (ff.V.)
AQARlGuS (mushroom) ‘.—Figure, 192 :
general characters, 191 : microscopic
structure, 193 : spore-formation, 193
Algas (a/^a, sea-weed), 169, 432
Alternation of Generations, meaning of
the phrase explained under Bougain-
villea, 248: Moss, 408: Fern, 429:
F2quisetum, 438 : Salvinia, 442 : Selagin-
ella, 446 : Gymnosperms, 460 : Angio-
sperms, 476
Ambnla'oral {amhulacrunt^ a walking
place) groove, 307: ossicle.s 308 ; system,
starfish, 309—313
AMCEB'A (afiocfiot, changing):— Figure,
2 ; occurrence and generm characters, r :
movements, 4, 9 : sp^ies of, 8 : resting
condition, 10: nutrition, xi : growth,
X3 : resj^iration, ty : metabolism, tj : re-
fmuction, 19 : immortality, 20 : conju
aation, 90 : death, 30, ax ; conditions of
uifet ax : animal or plant? 180
Amosb'oid movements, 4
Amosb'nla (diminutive of Amoeba), the
amoeboid germ of one of the lower or-
ganisms. 51, §4
Anab'OllBm (avafioKi^, that which is
thrown up). See Metabolism, construc-
tive.
Anaeroyio (a, not : di^p, air : /Sior, life),
applied to those microbes to which free
oxygen is unnecessary, 93
An'al (dntts, the vent) segmsnt, Poly-
gordius, 270
An'al spot, Paramoecium, 113
An'aEjtateg (AvderTOTor, from avacrrijvai,
to rise up) See Mesostates, anabolic.
Anatomy (dvareVi/w, to cut up), the study
of the structure of organisms as made
out by dissection, 289
Andrmoliun (di^p, a male : olxoc, a
dwelling), the collective name for the
male sporophylls in the flower of Angio-
sperms, 464, 471
Air GIOSl^RMS (ayyeioi', a vessel :
<rir<p/xa, seed) Figures, 462, 465, 467,
470, 472 : general characters, 461 :
structure of flower, 463 : reduction of
gamobium, 474 : pollination and fertiliza-
tion, 460, 475: formation of fruit and
seed, ana development of the leafy plant,
definition of, 179
Animals, classifleation of, 320
Antirtala and Plants, comjpari.son of type
forms, 176 : discussion ofdoubtful forms,
180
Animals, Protlsts, and Plants, boun
daries between artificial, 182
Annual Rings, 450
Anodonta (a not : o8ovir, a tooth). See
Mussel
Antenna {antenna^ a sail-yard), 325
Atttennary Gland. See Kidney
Antennnle (dim. of antenna), 325
490
INDEX AND GLOSSARY
Antberozo'id. See Sperm.
the vent), the posterior aper-
ture of the enteric canal, 270
APloal oell :--Penicillium. 190: Nitella,
ao8 : Moss, 403 : stem of Fern 418 : root
of Fern, 421 : prothallus of Fern, 425
Ap'ical cone, Fern, 418
A’pioal growth, 190,419
A'ploal mer'iftem, a mass of meristem
at the apex of a stem or root, 418,
Apjpen'dages, lateral ;-~crayfish, 321 : dog-
fish, 369
Arohegonlnm (apx^i beginning : 701/0$,
production), the name usually given to
the ovary of the higher plants
Aristotle, abiogencsis taught by, 96
Arteries, Crayfish, 337 ; Dogfish, 384
ArthrohranoUa(dp0pov, a joint : ppdyxta,
Anthroi^a, the, 306
Artlfloial reproduction of Hydra, 231
Asexual generation. See Agamobium.
Asexual reproduction. See Fission,
Budding, Spore.
Asparagin, 410
ASMmlla'tlon (assitniU^ to make like), the
conversion of food materials into living
protoplasm, 13
Ast'aous. See Crayfish.
Asterias. See StaHish.
Astoo rehere. 65
AtTropny (a, without : rpo^ij, nourish'
ment), a wasting away, 118
Auditory organ, Crayfish, 342 : Mussel,
363 ; Dogfish, 395
lurldo. See Heart.
Autom'atlsm (a{/r6/yiaro$, acting of one’s
own wiliX 10, 244
Axial Bundle, Moss, 403
Axial fibre, Vorticella, 129
Axil {axilla^ the arm-pit), 206
Axis, primary and secondary, 206
B
BAOILXUS (ddiiiiuM, a little staflf), 85
Figure, 8?
BAuTBRIA OSaxnJpioi/, a little staff) or
MIOBOB£S(p.txpo$, small : / 3 io$, life): —
occurrence, 82 : structure of chief genera,
84: reproduction, 87: nutrition, 89;
ferment'action, 91 ; parasitism, 93 : con'
ditions of life,^ 92 : presence in atmos-
phere, 102 : animals or plants? 183
BAOTBRinU termo (Figures) 83, 84
Baer, von. Law of Development, 43
Bamiacle^isese, supposed hetcrogenetic
production of, X03
Bark. See Cortex.
Bast. SeePhlo8fn.
Btimoinliial s^eiufiature, 8, xm
Btot W^iligi (jMsti 1^: ydramfi origin),
the origin of organisms from pre-existing
organisms, 96 : former belief in, 96 : early
experiments on, 97 : crucial experiment
on, 98
BiOl'Ogjr Oto$, life: Adyor, discussion),
the science which treats of living things
Bipinnaria, larva of Starfish, 317
BljMt'OCOele OSAao-rdr, a bud : xoiAov, a
hollow), the larval body-cavity, 205
Blood, Polygordius, 280 : Crayfish, 341 :
Mussel, 363 : Dogfish, 390
Blood-corpuscles: colourless, see Leuco-
cytes : red, 56, 300 : Figures, 57
Blood-vessels, Polygordius, 282 ; develop-
ment of, 302 : Starfish, 314 : Crayfish,
337 : Mussel, 360 : Dogfish, 384
Body-cavity. See Blastocoele and Coe-
lome.
Body-segments. See Metameres.
BOUaAINVILLBA (after L. A. de Bou-
gainville, the French navigator : —
Figure, 233. occurrence and general
characters, 234 : microscopic structure,
236 : structure of medusa, 339 : structure
and functions of nervoUs system, 242 :
organs of sight, 244 : reproduction and
development, 245 : alteration of genera-
tions, 248
Bract (dracteat a thin plate), 249
Brain Polygordius, 283 ; trochosphere,
296 : Crayfish, 341 : Dogfish, 391
Branch, Nitella, 2c6
Brstnch'la (/Spdyyia, gillX See Gill.
Branchial apenures. Dogfish, 368, 381,
383 .
Browne, Sir Thomas, on abiogenetic origin
of mice, 96
BUO'cal {hucca, the cheek) groove, Para-
moeciwm, 109
Bud, budding, Saccharomyces, 73 : com-
parison of with fis.sion, 73 : Hydra, 233
Bundle-Sheath. See Endodermis.
Buttercup, flower, 464 (Figure)
C
Galyp'tra (xoiAvirTpa, a veil), 407
Oalyx (KdAv^, the cup of a flower), the
outer or proximal whorl of the perianth in
the flower of Angiosperms, 464, 47X, 473
Gambium, 450
GanalS, r^ial and circular, medusa, 239
Oanal-oellB of ovary, 405, 426
Gap-cells of roots, 422
Garapaoe, Crayfish, 318
Carbon dioxlae, decomposition of by
chlorophyll bodies, 29
Cardiac division. See Stomach.
Gar'pel (xapir6$, fVuit), a female sporophyll,
^455. 456, 464, 467
GaxtUa^, 372
Gax'tilitt^ 372
OAXnXRTA ^avAd$, a siem : iptrta, to
^ ^P)i *75 (figure)
Cell ictlia, a closet or hut, from the first
INDEX AND GLOSSARY
491
conception of a cell having been derived
from the walled plant-cell, ‘.—meaning of
term, 6t : minute structure of (Figure),
62 : varieties of (Figure), 57
Oell-agmegate, meaning of term, 188.
CaU-OOlOIiy ; — temporary, Saccharoroyces,
72 : permanent, Zoothamnium, 135
OellHliyision, 65 (Figure)
Gell-fasion 302, 419
Cell-layer, 222, 273
Oell-membrane or wall, 10, 27, 63
Cell-multiplication and differentiation,
215 : Polygordius, 302 ; Fern, 419
CeU-plate, 67
Cell-protoplasm, 62
Ceirulose, compos*' tion and properties of,
28
Central capsule, Radiolaria, 152
Central particle or Centrosome (KcVr^i/,
centre : aaifia, the body), 65, 261 (Fig-
ure)
Ceph^alothor'az (xe^aXy, head : Ooipaf,
breast-plate), Crayfish, 318
Cerebral ganglion. See Brain.
Cerebro-pleural ganglion, Mussel, 362
CheUped (yijA^, claws: irotis, foot), 323
Chlorophyll (x^ojpof, green: ^uAAov, a
leaf), the green colouring matter of
plants, properties of 26, 31 : occurrence
in Bacteria, 87 : in Hydra, 228
Chrom'atln (voMua. a colour), the con-
stituent of the nucleus which is deeply
stained by dyes, 7, 63 : male and female
in nucleus of oosperm, 261
Chrom'atophoru colour:
to bear), a mass of proteid material im-
pregnated with chlorophyll or some other
colouring matter, 26, a6, 197, 207, 228
Chromosome ( itouuou colour : body),
65, 66, 261
Oil'lum (fiiittm, an eye-lash), defined,
: comparison of with pseudopod,
34, 52 : absence of cilia in Arthropoda,
Olf'lLy mOYOment, 25 : a form of con-
tractility, 33
emulate Inmsorla, 107
Cironlatory oigans, Polygordius, 282 :
Crayfish, 337: Mussel, 360: Dogfish,
384
C&assifioatlon, natural and artificial, 141 :
natural, a genealogical tree, 145
Cnid'OhlMt (kviBii, a nettle : fiXaarroi, a
bud), the cell in which a nematocyst
(f.v.) is developed, 227
Cmid'OW (mdif and ciiium\ the trigger-
hair” of acnidoblast, 227
tksleuterata, the, 305
dOSlome (ieoiA(*>/u.a, a hollow), the body-
cavity :— Polygordius, 270 : Starfish,
^ : Crayfish, 3«, 343 : Mussel, 355 :
Ai^sh, 37a: development of, Poly-
Mom^ail^rovided with a oedome, 373
Cmlomio epithelium. See Epithelium.
CcBlomio fluid, Polygordius, 278
Colloids (ifdAAa, glue : et8o«, form), pro-
perties of. 6
Colony, Colonial organism, meaning of
term, 135, 234: formation of temporary
colonies, Hydra, 231
Colnmel'la (a little column), 162
Com'missure {corumtsstira, a band), 279
Compound organism. See Colony.
(enm, together : cresco, to
grow), the un'on of parts during growth
Cone, an axis bearing sporophylls Equi-
setum, 436 : Selaginella, 444 : Gymno-
sperms, 452
00 'i^yig 9 i>'%ioJi{conjngdtio, a coupling), the
union of two cells, in sexual reproduc-
tion : — Amteba, 20 : Heteromita, 41 :
Paramcecium, 114 : Vorticella, 132 :
Mucor, 165: Spirogyia, igS : of ovum
and sperm, 260: monoecious and dioe-
cious, igg : comparison with plasmodium-
formation, 54
ConneotlYO, oesophageal, 283, 341
OonneotlTo tissue, 329. 369
Oontraotllo vao'uole {vacuus, empty) : —
Amoeba, 8, x6 : Euglena, 47 : Paramoe-
cium, III
C^ntraotil'ity {contractu, a drawing to-
gether), nature of, xo, 34: muscular,
*3°
Oontraotion, physical and biological, xo
Conus artinosus, 384
Cork, 449
Cork-oambium, 451
Corolla {cdrolla, a little wreath), the
inner or di.Htal whorl of the perianth in
the flower of Angtosperms, 464, 468, 471,
473
Corpusoles. See Blood corpuscles, and
Leucocytes.
Cortez, oor'tioal layer (cortex, bark),
Floweiing plants, 59, 448: Infusoria,
1X0, X26
Cotton-wool as a germ-fitter, 99
Cotyle'don (kotvAx^wi^, a cup or socket),
the first leaf or leaves of the phyllula
(g.v.) in vascular plants, 427
Cranium (xpavCou, the skull), 374
CRATFISn :— Figure. 319; general charac-
ters, 314, 3x5 : limited number and con-
crescence of metameres( Figure), 220: ap-
pendages (Figure), 322: exoskeleton,
319 : enteric canal (Figures), 322 : gills
(Figure), 318: blood-system (Figuie,)
335» 337 • kidney, 337 : nervous system,
319 : Muscles (Figures),' 327 : reproduc-
tive organs, 343 : development, 343
Creation (creo, to produce), definition of,
141 : illustrated in connecton with species
of Zoothamnium (Diagram^ 14*
Grogf-ferttlisatton : applied to the semial
process when the gametes spring from
different individuals, 199
492
INDEX AND GLOSSARY
Onrit'alloldf (lepvaroAAof, crystal : elSot
form), properties of. 6
Oatlole {fiuiiaila^ the outer skin), nature
of in unicellular animals, 45, 109: in
multicellular animals, 238
Cyst (Kiiflrrtf, a bag), used for cell-wall in
many cases, 10, 51
D
Dallinger, Dr. W. H , observations on an
apparent case of heterogenesis, 103
Pawchter-oellB. cells formed by the fission
or igcmmation of a mother-cell, 35, 67
DOfttn, phenomena attending, 20, 21, 166
Deoomposition, nature of, 6, 91
Dermal gillB. See Respiratory Cccca.'
Dermis (fitpfia, skin), the deep or connec-
tive tissue layer of the skin, 326^
Descent, doctrine of. See Evolution.
Detrelopment, meaning of the term, 43.
For development of the various types
see under their names
Dextrin, 113
Diastase, 81
Diast'Ole f8iao-reAA«d, to separate), the
phase of dilatation of a heart, contractile
vacuole, &c., zzz
DIATOMA'OEiB (fitartfivoty to cut across,
because of the division of the shell into
two valves), zss : Figure, Z56
Diat'omln, the characteristic yellow colour-
ing matter of diatoms, 154, Z55
Dlobot'omous (Bixorofitia, to cut in two),
applied to branching in which the stem
divides into two axes of equal value, 3x8
DtfTerentia'tlOn {flifferOy to carry different
ways), explained and illustrated, 34, no
DlSM'tion to arrange or digest),
the process % which food is rendered fit
for absorption, 12, : intra-^ and extra-
cellular, 229: contrasted with assimila-
tion. 230
Dlgest'lve gland, 335, 355. 382
Dimorpdi'ism, dimorph'ic (Hs, tufice:
form), existing under two forms,
35, Z36, 242, 438, 442 ,
DlCS'OlOUS ( 6 is, twice : oZico$, a dwelling),
applied to organisms in which the male
and female organs occur in different in-
dividuals, Z99
DIpyYBS (Minis, double): Figure, 250;
occurrence and general characters, 248 :
polymorphism, 249,
DlplOblatwiO (diirAeoe, double : fiXaaroi, a
bud), two-layered : applied to animals in
which the body consists of ectoderm and
endoderm, 236: derivation of diploblas-
tic from unicellular animals, a6z
DiraoUve sphere see also Astrosphere
DlSO^ Vorricella, xa8
Diipanilily means of : in internal parasite,
;34 : in fixed organisms, 133, 234
Distal, the end furthest from the point of
attainment or organic ba.se, xafi
Distrltmtion of food-materials in a
complex animal, 278: in a complex
plant, 409
DlYorganoe of obaracter, 145
DlTisloii of pb^slologioal labour, 34
DOQFISfl Figure, 367 : general charac-
ters, 368 ; exoskeleton, 369 : endo-
skeleton (Figures). 372 : enteric canal
(Figures), sSt : gills. 383 : blood-system
(Figures), 384 : kidney, 396 : gonads,
3 q 6: nervous system and sense organs
(Figure), 391 ; development (Figure),
397
Dry-rigor, stiffening of protoplasm due to
abstraction of water, 21
E
E c ^SlS (exfiven.^, a slipping out), 325
Ecblnodermata, the, 305
Ect'Oderm (exrds, outside ; ^fopa, skin),
the outer cell-layer of diploolasttc and
triploblastic animals. 222, 275
Bot'osaro (exrof, outside : <r<tp^, flesh), the
outer layer of protoplasm m the lower
unicellular organi.sms, distinguished by
freedom from granules, 4
Egest'lOXl {egSrOy to expel), the expulsion
of waste matters, Z2
Egg-oell. See Ovum.
Slater, 436
Em'bryo (epfipvov, an embryo or foetus),
the young of an organism before the
commencement of free existence.
Emliryo-sao. See Megaspore. ^
Enoysta'tion, being enclosed in a cyst
,
End'oderm (SvSov, within; SeppOf skin),
the inner cell-layer of diploblastic and
triploblastic animals, 222, 228, 275
Endodermls, 418
End'oderm-lamella, Medusa, 240
Sndog'enous (evSov, within : yiyvopMy to’
come into being), arising from within,
e.g. the roots of vascular plants, 422
Eadopbragmal System (ev8oi/, within:
^pdypMy protection), 321
EndO]radlte (Ivfiov, within , iroi;c, foot), 323
Bnd'osaro (HvSovt within: <rdp$, flesh),
the inner, granular protoplasm of the
lower unicellular organisms, 4
Endoskereton (evSov, within^ and skeleton^
from zrxcAAtt, to dry), the internal skele-
ton of animals, 372
End'o^rm(<v6ov, within : tmdppMi seed),
nutrient tissue formed in the megaspore
of Phaqerogamsj 459, 476
conversion of potential into
kmefic, t5 : source of, tn chlcxophyll*
containing organisms, 32
INDEX AND GLOSSARY
493
Enter'to (emepov, intestine), oanftl, the
entire food-tube from mouth to anus
Polygordius, 970, 976: Starfish, 31a:
Crayfish, 332 : Mussel, 355 : Dogfish, 381
EnVeron or Enterlo oa^ty, the simple
digestive chamber of diploblastic ani-
mals, 222
E]^demi'i8 (eni upon ’• St'pfia, the skin) :
in animals synonymous with deric epi-
thelium (y.t/. , under Epithelium) : in vas-
cular plants a single external layer of
cells, 416, 420
EpipoditO (eirt, upon ; rrovt, foot), 324
Epi^Stoma (eiri, upon : crro/ia, month), 321
Epitherial cells : columnar, 58 : ciliated,
Epi^herium (eiri, upon : Biikn, the nipple),
a cellular membrane bounding a free
surface, ^4 : GOBlomlo, 274 : deric,
273 ; enteric, 274
EQtnSE'TUM {eg HUS, a horse: seia, a
bristle) ‘.—Figures, 435, 437 : general
characters, 434 : cone an<l sporophylls,
436 : male and female prothalli, 437 : al-
ternation of generations, 438
Equivocal generation. See Abiogenesis.
EuGLEN'A ( wykrivoii bright-eyed)
Figure, 45 : occurrence and general
characters, 44 : movements, 44 : struc-
ture, 45 : nutrition, 46 : resting stage.
47 : reproduction, 48 : animal or plant h
180
Euglen'oid movements, 0
Ev’olution {.evolve, to roll out), organic :
definition, 143 : illustration of in connec-
tion with species of Zoothamnium (Dia-
gram). 144
EzoreTion {excemo, to separate), the
separation of waste matters derived from
the destructive metabolism of the or-
ganism, 16, 281
Ezog'enons (e{, out of: yiyvop.ai, to come
into being), arising from the exterior,
leaves, 422
Exopodlte (e^w, outside ; irovs, foot), 323
Ezoskereton (e^o), outside, and skeleton,
from (rxeA\(i> to dry), the external or
skin-skeleton . GUtlOlUar, 238, 273 : der-
308, ^27, 350, 369
Bye, Crayfish, 342 : Dogfish, 394 :
Bye-epotg or Ooeril Medusa, 244 :
Polygordius, 296
Eye-italks, 321
Fsecee ifaejt, dregs), solid excrement,
consisting of the undigested portions of
the food, 16
Ferm'ent {fermentum, yeast, from fer-
viOj to DoW or ferment), a substance
which mduces fermCBta'tlQn, i,e. a
definite chemical change^ in certain sub-
stances with which it is brought into
contact, without itself undergoing
change : unorgaatoed and organised
ferments 80 : • dlcohollo, 76 : ace-
tous, 9t : dlastatlo or amylolytlo, 81 :
lactic, 91 : peptonizing or proteomlo,
8x : putrefactive, 91 : ferment-celTs of
Mucor, 168
FERNS Figures, 414, 424 : general
characters 412 *. histology of stem, leaf,
and root, 415 : nutrition, 422 : spore-
formation, 422 : prothallus and g^onads,
425 : development, 426 ; alternation of
generations, 429
Fertiliza'tlon (/ertills, ^ bearing fruit) ;
the process of conjugation of a sperm or
sperm-nucleus with an ovum, whereby
the latter is rendered capable of develop-
ment : a special case of conjugation
(g.v.), 199 : details of process, 260 : in
Vauchena, 173 : in Gymnosperms, 458,
in Angiosperms, 475
Filtering air, method of, 99
Fins, Dogfish 369
Fission {Jlssio, a. cleaving), Simple or
binary, the division of a mother cell
into two daughter-cells : in Amoeba, 19 ;
Heteromita, 40 : animal- and plant-cells
generally, 65 : Paramuecium, 714 :
Vorticella jji
Fission, multiple, the division of a
mother cell into numerous daughter-
cells in Heteromita, 42 : Protomyxa,
51 : Saccharomyces, 74
Fission, process intermediate between
simple and multiple, Opalina, 124
Flagellum. See Cilium.
Flag'ellate Infusoria, 107
Flagell'ula (diminutive of flagellum), the
flagellate germ of one of the lower
organisms (often called zoospore.s, 51, <4
Flagell'um (flUgellutu, a whip) : defined,
25 : transition to pseudopod. 52, 228
Floral receptacle, the abbreviated axLs of
an angiospermous flower 464, 471, 473
Flower, a specially modified cone {g.v.),
' having a shortened axis, which Dears
perianth-leaves as well as sporophylls,
463 : often applied to the cone of Gymno-
sperms, 452
Food-current, Mussel, 359
Food-vacuole, a tempjrajry space in the
protoplasm of a cell containing water
and food-particles, ii, xx2
Foot ; of Mussel, 349 : of phyllula of fern,
F^^UIONIFERA {foramen, a hole \fero
to bear), 1^8 : Figures, 149, 150, xsx
Framenta^tion of the nucleus, lao
Fruit of Angiosperms, 476
Fimt/tlon (functio, a performing), mean-
ing of the ternif 9
G
Qam'ete {yap.iw, to marry), a conjugating
cell, whether of indeterminate or deter-
494
INDEX AND GLOSSARY
minate sejc Hcteromita, 41 : Mucor,
- 156 : Spirogyra, 198 : Vaucheria 173
Oamobluni (ya/iof, marriage : lire)>
the sexual generation in organisms ex-
hibiting alternation of generations ( <7.7/.) :
regressive subordination of, to agamo-
ium in vascular plants, 429, 440, 444,
^458,474, ,
Ganglion (yayyXtoi^, a tumour), a swelling
on a nerve-cord in which nerve-cells are
accumulated, 341
Gastric juice (-yaerr^p, the stomach), pro-
perties of, 12
Gastric mill, 334
Gastrolitll {yacm/jp, stomach : \i 0 or,
stone), 334
Gast'rula (diminutive of yaanjp, the
stomach), the diplohlastic stage of the
animal embryo in which there is a diges-
tive cavity with an external opening:
characters and Figure of, 295 ’.contrasted
with phyllula, 428
Gemma'tion {gemma, a bud). See Bud-
ding.
Genera'tion, asexual, See Agamobium.
Generation, sexual. See Gamohium.
Genera'tioncL Alternation of. Sec Al-
ternation of generations.
Generative oml, 455
Generative nucleus, 474
Gen'erallzed, meaning of term, 140
Ge'nus {genus, a race), generic name,
generic characters, 8, 139
Germ-filter, 99
Ger'minal spet, the nucleolus of the
ovum, 257
Germlna'tlon {germinatio, a budding),
the sprouting of a spore, zygote, or
oosperm to mrm the adult plant ; for
germination of the various types see
under their names.
Gill, an aquatic respiratory organ. 335, 357,
383
Gland {^glans, an acorn), an organ of
secretion {q-v.) : gland-cells, 228,
278
GlOChidlum, larva of Mussel, 365
Gon'ad (yot'os, offspring, seed), the essen-
tial organ of sexual reproduction,
whether of indeterminate or determinate
.se.v, /.I?, an organ producing either un-
differentiated gametes, ova, or .sperms ;
under the various types, and espe-
cially 172, 198, 21X, 290
Gon'oduct {^mad and dilco, to lead), a
tube carrying the ova or .sperms from the
gonad to the exterior, 292
Grapplxm-llnes, Diphyes, 249
Green gland. See Kidney
Growing point : Nitella, 208 : Moss, 403 ;
Fern, 418: Gymnosperms, 452
Grcwtii, 13
Guard-oelle of stomates, aax
Gullet, the simple food-tube of Infusoria,
47, ixo : or part of the enteric canal
the higher animals, 277^
QYM'KwPERMS {yvpivoi, naked : ervepfin
seed) : Figures, 449, 453, 455, 456, 457 :
general characters, 447 : structure and
growth of stem, 448 ; structure of cones
and sporophylis, 452 : reduction of gdmo-
biumfprothalli and gonads), 460 : pollina-
tion and fertilization, 458 : formation of
seed and development of leafy plant, 459
Gynoeolum (ywrf, a female : oIkov, a
dwelling), the collective n.ame for the;,
female sporophylls in the dower ot
Angiosperms, 465, 471
H
Hssm'atoolirome (aT^a, blood : vpw/ia,
colour), a red colouring matter allied to
chloroyhyll, 26
HiEMATOOOO'CUS atjua, blood : kokxov
a berry). — Figure, 24: general characters,
23 : rate of progression, 23 : ciliary move-
ment.s, 25, 33 t colouring matters, 26 :
niotile and stationary phrases, 28 : nutri-
tion, 28 : source of energy, 30 ; reproduc-
tion, 35 : dimorphism, 35 : animal or
plant? 180
Hmmoglob'in (aXfia, blood : glohns, a
round body, from the circular red cor-
puscles of human blood), 58 : properties,
and functions of, 280
Head-kidney : trochosphere, 297
Heart Crayfish, 337 : Mussel, 360 :
Dogfish, 384
Heat, evolution of by oxidation of proto-
plasm, 17
Heat-rigor {rigor, stififness), heat-stiffen-
ing, 21
Heliotropism, 168
Hepato-paucreas (i^n-op, liver : n-ayxpeas,
sweetbread), 335 i
Heredity {hereditas, heirship), 147
Hermapu'rodlte {ipneuf^oSlTo^, from
Hermes and Aphrodite). See Monoecious.
Heterogen'esls (ertpo^ , different : yevemt,
origin), meaning of term, 102 : supposed
cases of, 103 not to be confounded with
metamorphosis or with evolution, 104
HETEBOluTA (crepov, different : ftirov,
a thread) Figure, 38 ; occurrence and
general characters, 36 ; movements, 37 :
nutrition^ 37 : asexual reproduction, jo :
conjugation, 41 : developmeiif and life-
history, 42, 43 : animal or plant ? x8x ^
High and low organisms^ 106
Higher (triploblastic) animals, uniformity
in general structure of 304
Higher (vascular) plants, uniformity tii
general structure of, 431 A
Histol'ogy CioTTtov, a thing woven : Aoyo^
a discussion), minute or microscopic
anatomy, 289
INDEX AND GLOSSARY
495
ffolophyt'io (oAof , whole : ^vr 6 v, a plant),
nutrition, defined, 31
Holozolo (0A09, whole : ^taov, an animal),
nutrition, defined, 31
Homopien'esis (6fi6« , the same:
origin), meaning of the term, 103
HoniorogOUS(6fi6XoYOs, agreeing), applied
to_ parts which have had a common
origin, 343
Homomorphism homomorphic (&/xdf,
the same : fioppij, form), existing under
a single form, 139
Host, term applied to the organism upon
which a parasite preys, 123
HYDRA (ifopo, a water-serpent) ; Figures,
219, 223, 225, 232: occurrence and general
characters, 218 : species, 220 : move-
ments, 220 : mode of feeding, 221 : m.cro-
scopic structure, 222 : digestion, 229 :
asexual, artificial, and sexual reproduc-
tion 230 : development, 233
Hydr'anth (vSpa, a water-serpent : dv8ov,
a flower), the nutritive zooid of a hydroid
polype, 236
Hydzuid (i/Spa, a water-serpent : eidot,
form) Polyp®S (iroAvirouv, many footed),
compound organisms, the zooids of which
have a general resemblance to Hydra,
234
H3rper'trophy (vtrdp, over : Tpo(f>v, nourish-
ment), an increase in size beyond the
usual limits, 118
HjTph'U (t/^oUvia, to weave) applied to the
separate filaments of a fungus : they
may be mycelial (see mycelium ), sub-
merged, or aerial : IWucor, 160: Pern-
cillium, 185
Hyp'odermiS (uird, under : 64 pp.a , skin),
Fern, 413, 416
Hypostome (vn 6 , under : o-rdfia, mouth),
220, 236
InSOla'tion (imoU, to place in the sun),
exposure to direct sunlight, 94
Integ'ument {int^gttmentum^ a covering)
of megaspore : Gymnosperms, 456 :
Angiosperms, 466
luter-oeuular spaoes, 415
Inter»mu8cular plexus (irAextD, to twine),
285
Intemode {inters between: nodus^^ a
knot), the portion of stem intervening
between two node.s, 205
Intersti'tlal {interstUUim^ a space be-
tween) celu, Hydra, 224: growth,
Spiroeyra, 198
Xntest'me {in/estlnus, internal), part of
the enteric canal of the higher animals,
277
lUtUS'SUSCep'tlon (tnius, into: susa'ph,
tp take up), addition of new' matter to
the interior, 13
Iodine, lest for starch, 27
Irritabirity ifrritabiiis, irritable, the
property of responding to an external
stimulus, TO
J
Jaws ; Crayfish, 324 : Dogfish, 368, 375
K
Karyokines'is (xapvov, a kernel or nu-
cleus : KiKijo'if, a movement), indirect
nuclear division, 67
Katab'olism (Karo^oAi), a laying down),
18. See Metabolism, destructive.
Kat'astates (Karoim^vai, to sink down),
18. See Mesostates, kataholic.
Kidney :~Crayfish, 337: Mussel, 359:
Dogfish, 396
I
L
Immortality, virtual, of lower organisms,
21
Income and expenditure of protoplasm,
18
Individual. See Zooid.
Individuation, meaning- of the term, 230,
252
IndttS'ium (iftdusmm, an under-garment,
mflores'oenoe {ftoreseo. to begin to
flower), an aggregation of cones or
flowers, 454
InfUSOr'i^ (so called because of their fre-
quent occurrence in infusions), xcn
Ingesta {ingiro, to put into) and Egesta
(ogiro, to expel), balance of, 32
In^^atlon (ingifo, to put into), the taking
in of solid food, 11
Labial palps, Mussel, 355
Larva, the free-living young of an animal
in which develojpment is accompanied by
a metamorphosis, 293
Larval sta^s, significance of, Potygor-
dius, 303 * \
Leaf, structure of:— Nitella, 205, 207^:
Moss, 403 : Fern, 420 : limited growth
of. 2ZX
Leaflet, Nitella, 207
Leg, Crayfish, 324
Ziept'0thriX(AcirTof, slender : 9 pl(, a hair),
nlamentous condition of Bacillus, 89:
Figure, 87
Leuo'ooyte (Aci;xd« white : xvToe a hollow
vessel, cell), a colourless blood corpi:^le :
—structure of, in various animals
(Figures), 57; ingesUon of solid par*
498
INDEX AND GlXJSSAkY
S04, aog, aist, $74: occurrenec ami
general cbajacters,^ 203 : microscopic
structure, 206 : terminal bud, 208 : struc-
ture and development of eonads, 206,
211: development, 216; alternation of
generations, 217
Node inodus, a knot), the portion of a
stem which gives rise to leaves, 205
NoVOOhOXd (vAtov, the back: x®P^^» ^
Nlioel'fttl^^l^iininurive of nurler/s, the
name formerly applied), 456, 466
Ntt<d 0 ar dtviiioii, mdireci : 64 (Figure)
dhroot, 67
Nttotoar mombrane, 63
Nuoledr aap, 7, 63
Nuolaarspmdle, 66
NllOlB'Olttl (diminutive of nucleus), 8f 63
Mtt'OldUt(nt2r/4rMr, a kernel), minute stnic-
turc of, 63 ; Amoeba, 7, 8 : Paramoeciuin,
III, X14 ; Opalina, 121 : Vorticella,
128 : Nitella, 208, 21 x : fragmentation
of, 120
Ntt0l6UI» sooondary, of megaspore, An-
giosperms, 474
Nutrient solution, artificial, principles of
construction of, 77,
Nutrition A mceba (holo/oic), tr; Hse-
matococcus (holophytic), 28: Hetero-
mitit (saprophytic), 37: Opalina (type of
internal parasite), 123 : Mucor 167 :
PenicilUum, 190 : Polygordius (type of
higher animals), 270, 279 : Moss [ty^
ofnigher plants), 408
OoeVlttf (dcfUus, a little eye), structure
and functions of, Medusa, 240, 244
<BS0pll'aga8 (Qi(r^aY 07 , the gullet). 5 ?ce
Gullet.
(Nflaotory organ. Crayfish, 342 : Mussel,
363; Dogfish, 394
Oininatldeum (aim. of op.ft.a, eye), 342^
Ont(^'eny (owos, being . yeVeerts, originX
the development of the individual :
a recapitulation of phylogeny (?.t/.),
T 4 fi
GOI^'Olla (enfv, an egg: yeVeerx?, originX
the development of an ovum from a
primitive sex>ceU, 256
Od^tA’iUBl (iMV, egg : yovM, produc*
tion), the name usually given to the
ovary (g.p,) of many of the lower plants.
GMpOnU (wdv, e^: crirtp/aa, seed), a
i^ote (p.t'.X formed by conjugation of
line ovum and sperm: a unicellular
embryo, 173* *fio; origin of nucleus of,
a6x
OlUq^are an egg: <r^«upa, a sphere), ^
a name frequently given to the ovum
of plants,
f()Mpore an egg: (rxropd, a seed), a
name frequently applied to the oosperm
ia,v.)of plants.
OPALIN'A (from its opalescent appear*
ance) '.—Figure, 12a : occurrence and
general ^ characters, 121 : structure
and division of nuclei, 121 : parasitic
nutrition, 123: reproduction, 124: means
of dispersal, 124: development, 123
Opt'tmmn (optimns, best) temperature for
amrel}oid movements, 21 : for sapro*
phytic monads, 40
Organ (opyavov, an instrument), a portion
•)f the body set apart for the peifirmaju e
of a particular function, 288
Or'ganism, any living thing, whether
animal or plant, 5
Osphtadlumfocr^paivo^ou, to smell), 363
Oss'iCle (diminutive of ds, a bone), 308
Ov'ary {dvum. an egg), the female gonad
or ovuin-proaucing orga 1 ; see under the
variou.s types and especially Vaucheria,
y2: atrophy of, in Arigio.sperms, 474.
The name is also incorrectly applied to
the venter of the pistil of Angiosperms,
Ov'liuot an egg : duco^ to lead),
a tube conveying the ova from the o\ary
to the exterior, 292
Ov'um {fUvnmy an egg), the female or
megagamete in its highest state of dif*
feren nation : general structure of, 68 :
minute structure and maturation of, 256;
see also under the various types and
especially Vaucheria, 173 : formation of,
in Angiosperms, 474
Oy'ulO (diminutive of cnium), the name
usually ^plied to the megasporangium
(^.7/.) of Phanerogams.
Oshdaiion of protoplasm, 15
OXTTRIOH'A (ofvs, sharp : a hairX
120 (Figure)
P
Pallium See Mantle.
P«Up (palpOf to stroke), Crayfish, 325*.
Mussel, 355
Paklcreap8(Trayicpe'o4, sweet^ad), 382
PANPORINATFigufeX 262
Param'ylum (mxpd, beside : afiUXov, fine
meal, starch), 46
PiJtAHCBPluM Figures, 108, 115:
structure, 107: mode of feeding, 112:
asexual reproduction, 114 : conjugation,
114
Par'asite, parasitism (napatriTo^, one
who lives at another’s table) ; —Bacteria,
92 : Opalina, 123 : Mucor, 167
Paren'CA^ma (trapiyxvfia, ^ anything
poured in beside, a word originally u.sed
to describe the substance of the lungs,
liver, and other soft internal organs),
applied to the cells of plants the lengthof